Effects of Trypsin on Binding of t3h]epinephrine and t3h]- Dihydroergocryptine to Rat Liver Plasma Membranes

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1 THE JOURNAL OF BOLOGCAL CHEMSTRY Vol. 255, No. 12, ssue of June 25, pp ,1980 Prrnted in U.S.A. Effects of Trypsin on Binding of t3h]epinephrine and t3h]- Dihydroergocryptine to Rat Liver Plasma Membranes EVDENCE FOR NTERCONVERSON OF BNDNG STES* (Received for publication, January 18, 1980) Mahmoud F. El-Refai+ and John H. Extong From the Laboratories for the Studies of Metabolic Disorders, Howard Hughes Medica! nstitute, and the Department of Physiology, Vanderbilt University School of Medicine, Nashville, Tennessee Treatment of liver plasma membranes with trypsin at low concentrations (1 to 2 pg/mg of protein) caused a 3- to 4-fold increase in a-specific THepinephrine binding.the change was due to an increase in the number of high affinity binding sites, with no change in the dissociation constant. With increasing trypsin concentrations, the dissociation constant was decreased and there was a progressive loss of binding. Elastase, papain, and thermolysin caused similar effects, whereas thrombin, leucine aminopeptidase, phospholipase Az, phospholipase C, phospholipase D, and detergents did not cause an increase in [3H]epinephrine binding. The increase in epinephrine high affinity binding sites was correlated with a loss of high affinity r3h]- dihydroergocryptine binding sites which also bind [3H]epinephrine with low affinity (El-Refai, M. F., Blackmore, P. F., and Exton, J. H. (1979) J. Biol. Chem 254, ). ncubation of membranes with the LY blockers dihydroergocryptine (50 m) and phenoxy- benzamine (20 n ~) prior to protease treatment diminished the increase in [3H]epinephrine binding induced by trypsin (1.5 pg/mg). The concentration dependence and time course of trypsin actions on 70 n~ [3H]epi- nephrine binding and 10 n~ [3H]dihydroergocryptine binding are consistent with a trypsin-mediated conversion of low affinity epinephrine binding sites to high affinity epinephrine binding sites. Using [:'H]catecholamines and ['Hldihydroergocryptine, we have shown previously that in rat liver plasma membranes, there exist two types of a-adrenergic binding sites (1). One type of binding site binds preferentially, but not exclusively, a-adrenergic agonists and is believed to be the physiologically important receptor (1). The other type binds a-adrenergic antagonists with very high affinity and a-adrenergic agonists with much lower affinity (1). ts function is yet to be determined. n the course of looking for chemical differences between the two types of binding sites, we have shown that GTP causes inhibition of ['Hlepinephrine binding, while causing no change or slight increase in ["Hldihydroergocryptine binding (1). We have also found that both types of a-adrenergic * The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. $ Recipient of National nstitutes of Health Postdoctoral Fellowship AM Senior nvestigator, Howard Hughes Medical nstitute. binding sites are of the same subclass, i.e. cy] (2). Treatment of plasma membranes from rat liver with crude collagenase (3) or plasma membranes from rat embryo fibroblasts or rat ovaries with trypsin (4-6) has been reported to cause 2- to 3-fold increases in the basal activity of adenylate cyclase as well as in the activities stimulated by catecholamine or NaF. t has also been found that protease inhibitors can selectively block the stimulation of adenylate cyclase by human chorionic gonadotropin and proteolytic enzymes (4, 5). These findings are of interest in view of the notion that proteolytic enzymes play a role in a variety of physiological functions of a regulatory nature (6, 7). Examples of this type of regulatory mechanism include those enzymes and hormones which are synthesized in a precursor (physiologically inactive) form which is then made active by specific and limited proteolysis (6-8). n the present study, we have examined the effects of proteases and phospholipases on the a-adrenergic binding of ['Hlepinephrine and ["Hldihydroergocryptine to liver plasma membranes. The results suggest the possibility of interconversion between the binding sites with high affinity for [SH]dihydroergocryptine and the binding sites with high affinity for ['Hlepinephrine. The possible implications of these findings with regard to the regulation of a-adrenergic receptors are discussed. EXPERMENTAL PROCEDURES Materials-(-t)-[7-"H]Epinephrine (+)-bitartrate (15.5 Ci/mmol), and [9,10-3H]dihydroergocryptine (24 to 25 Ci/mmol) were from New England Nuclear. The following materials were from Sigma: trypsin (bovine pancreas type 1111, soybean trypsin inhibitor, thermolysin (protease type X) from Bacillus thermoproteolyticus, rokko, papain (type V) from papaya latex, elastase (type 111) from porcine pancreas, leucine aminopeptidase (type 111) from hog kidney, thrombin (grade ), phospholipase AL' from Naja naja, phospholipase C (type X) from Clostridium perfringens, phospholipase D (type 111) from peanut, and fatty acid-free bovine albumin. Other chemicals were those described previously (). Methods-The binding assays previously described in detail (1) were used with minor modifications. A typical assay contained 150 pl of membranes (approximately 2 mg/ml) suspended in 50 mm Tris buffer, ph 7.5, containing 0.8 mm ascorbate, 3 m~ catechol, 5 mm MgCls, 10 ( r (&)-propranolol, ~ and radiolabeled ligand in a final volume of 230 pl. Specific binding was defined as the difference between binding of the radioligand in the absence and presence of 10 p~ phentolamine. The incubations were carried out at 25 C for 30 min. The bound and free radioligands were separated by adding 100 pl of the incubation to 4 ml of 50 mm Tris buffer, ph 7.5, containing 0.8 mm ascorbate and 3 mm catechol at room temperature and rapidly filtering this under vacuum through Whatman GF/C glass filters. The filters were then rapidly washed twice with 8 ml of the above dilution buffer. The use of buffer at room temperature or at 4 C for dilution of sample and washing of filters causes no significant difference in specific binding measurements (1). Furthermore, the utilization of 5853

2 5854 Effects Trypsin of on a-adrenergic Binding centrifugation through 1% albumin at 4 C as a means of separating bound and free ligands yields similar results (1). Trypsin and other proteases were freshly dissolved for each experiment in 10 mm sodium acetate, ph 4, and then diluted appropriately in 10 mm potassium phosphate (ph 6.8) containing 1 mm EDTA immediately before the start of relevant experiments. Trypsin treatment was begun by adding trypsin solution to the incubation system and terminated by adding trypsin inhibitor at a concentration equal to 10 times that of trypsin. Potassium phosphate buffer, ph 6.8, was added to control tubes instead of trypsin. Protein was determined by the method of Lowry (9) with bovine albumin as standard. RESULTS Effects of Pretreatment of Liver Plasma Membranes with Proteases and Phospholipases on L3H/Epinephrine Binding-Fig. 1 shows the effects of treatment of plasma membranes with various concentrations of trypsin on the subsequent binding of ['Hlepinephrine. t is seen that the effects of trypsin pretreatment depended on the concentration of the enzyme. At low concentrations (1 or 2 pg/mg of protein), there was a consistent large increase in ["HJepinephrine specific binding. At higher concentrations, the increase became progressively less. Whereas pretreatment with 2 pg of trypsin/mg of protein for 30 min at 25 C caused an approximately 4-fold increase in ["H]epinephrine specific binding, pretreatment with 150 pgof trypsin/mg of protein was without apparent effect. There was also a consistent decrease in nonspecific binding with trypsin treatment. The increase in catecholamine specific binding, however, cannot be ascribed to the conversion of nonspecific binding to specific binding. This is because the loss of nonspecific sites with trypsin was not large enough to account for the increase in specific ["Hlepinephrine binding and because an increase in the total binding was observed as a result of trypsin treatment. Table shows the effects on ["Hlepinephrine binding of pretreatment of plasma membranes with various proteases for 30 min at 25 C. t is seen that the endopeptidases elastase, papain, and thennolysin caused increases in ["Hlepinephrine specific binding comparable to that caused by trypsin. On the other hand, the two exopeptidases thrombin and leucine aminopeptidase did not increase the binding or were inhibitory at high concentrations. The effects of three phospholipases on ['Hlepinephrine binding are shown in Fig. 2. Both phospholipase A:! and phospholipase D inhibited [3H]epinephrine binding at rela- \ 5001 i OJ TRYPSN CONCENTRATON (yg per mg protein) FG.. Effects of trypsin on the specific binding of 13H]epinephrine to liver plasma membranes. ncreasing concentrations of trypsin were incubated with liver plasma membranes for 30 min. Trypsin inhibitor was then added and assays of specific binding of 50 nm [3H]epinephrine were commenced as explained under "Methods." The measurements shown are means of four determinations. The experiment was repeated twice. The plasma membrane concentration in these and subsequent experiments was approximately 2 mg/ml. TABLE Effects ofproteolytic enzymes on the specific binding off Hepinephrine to liver plasma membranes Membranes were treated with low and high doses of several proteolytic enzymes for 30 min at 25 C before the binding of 50 nm ['Hlepinephrine was commenced. The measurements shown are means of four determinations and the experiment was repeat,ed once. The values in Darentheses refer to the enzvme concentrations. ['Hlepinephrine binding (% of control) Enzyme Treatment with low Treatment with high dose of enzvme dose of enzvme Endopeptidases Elastase 125 (2.5 pg/ml) 400 (25 pg/ml) Papain 165 (2.5 pg/ml) 320 (25 pg/ml) Thermolysin 195 (2.5 pg/ml) 485 (25 (rg/ml) Trypsin 250 (2.5 pg/ml) 200 (25 pg/ml) Exopeptidases Thrombin 100 (0.05 unit/ml) 100 (5.0 units/ml) Leucine aminopeptidase 100 (8 pg/ml) 40 (80 pg/ml) " ENZYME CONCENTRATON (U/rnll Frc. 2. Effects of phospholipases At, C, and D on VHepinephrine specific binding. ncreasing concentrations of phospholipases were added 10 min prior to the binding assay of 50 nm ["Hepinephrine. [3H]Epinephrine binding is shown as a percentage of control without enzyme. Values shown are means of four determinations and experiments were performed at least twice. The letters on the figure refer to the type of phospholipase used. tively low concentrations. n contrast, phospholipase C caused minimal changes at comparable activities. The effect of phospholipase A? was not due to release of fatty acids since inclusion of 5% albumin did not modify that effect. Treatment of membranes with low concentrations (0.005% to 0.1%) of detergents (Triton, digitonin, and Lubrol) did not increase ['Hlepinephrine specific binding and higher concentrations were inhibitory (data not shown). Time Course of Effects of Trypsin Pretreatment on Binding of [3H]Epinephrine and [3H]Dihydroergocryptine-The time courses of the effects of two concentrations of trypsin (2.2 and 22.0 pg/mg of protein) on [3H]epinephrine and [3H]dihydroergocryptine binding are shown in Fig. 3. The time course of trypsin action on [3H]epinephrine binding (upper panel) was biphasic. A first phase of increased ['Hepinephrine binding was followed by a phase of declining binding. The initial increase induced by pretreatment with trypsin at 22.0 pg/mg of protein was greater than that caused by 2.2 pg/mg of protein. However, pretreatment with 2.2 pg/

3 Effects a-adrenergic of Binding on Trypsin 5855 mg of protein resulted in a higher level of binding than that caused by pretreatment with 22.0 pg/mg of protein. These results suggest that trypsin causes a time- and dose-dependent destruction of the receptors in addition to a time- and dosedependent increase in their activity. The lower panel of Fig. 3 shows the time come of the effects of trypsin pretreatment on ["Hldihydroergocryptine binding. This binding was consistently decreased in a timeand dose-dependent manner. t was also observed that the nonspecific binding of both [3H]epinephrine and ["Hldihydroergocryptine was drastically decreased by trypsin pretreatment. Undetectable values for this binding were reached by 5 min. This time course suggests again that the increase of ['Hlepinephrine binding was not the result of conversion of the nonspecific binding component to specific binding. Concentration Dependence of (3ff]Epinephrine Binding to Plasma Membranes Pretreated with Trypsin-Fig. 4 (upper panel) shows the concentration dependence curves for ["Hlepinephrine binding to control membranes and membranes pretreated with trypsin at 5 pg/mg of protein for 20 min. The lower panel shows the Scatchard analyses of these data. For control membranes, Scatchard analysis yielded a dissociation constant (KO) range of 71 to 76 nm and a maximum number of binding sites (Bmax) equivalent to 205 to 223 fmol/mg of protein. n the case of trypsin-treated membranes, Scatchard analysis yielded a K, range of 31 to 38 nm and a B, equivalent to 496 to 527 fmol/mg of protein. n other words, the treatment caused a 2-fold increase in epinephrine affinity and a 2.5-fold increase in the number of binding sites as assessed by Scatchard analysis. The time course of trypsin action (Fig. 3) suggested the occurrence of at least two processes during the trypsin treat- r ['H EPNEPHRNE CONCENTRATON (nm), (BOUND)/(FREE) FG. 4. Effect of trypsin on the concentration dependence curve for specific binding of [3H]epinephrine. Liver plasma membranes were untreated (A) or treated with 5 pg of trypsin/mg of protein (0) for 20 min at 25 C. The lowerpanel shows the Scatchard plots derived from these data. The measurements are means of four determinations and the data shown are averages of three experiments. t 2O0 TRYPSN TREATMENT r PRE-TREATED POST-TREATED ph] FG. 5. Effects of trypsin treatment before and after ligand binding on 70 M [3H]epinephrine and 10 n~ [3H]dihydroergocryptine specific binding. Membranes were treated either before binding (pretreatment) or after binding (post-treatment) with 1.5 pg of trypsin/mg of protein for 20 min. Measurements are averages of four determinations. Data shown are from one experiment which was repeated once with similar results. L E ' TME (mln) FG. 3. Time course of the effects of trypsin pretreatment on the binding of 50 M r3h]epinephrine (upper panel) and 5 n~ [JH]dihydroergocryptine(lowerpanel). Liver plasma membranes were treated with two concentrations of trypsin, 2.2 pg/mg of protein (A) and 22 pg/mg of protein (O), for the designated times. Trypsin action was stopped byadding trypsin inhibitor and the assays of radioligand binding started as explained under "Methods." Data shown are the averages of four determinations and the experiment shown is representative of two similar experiments. The solid curues refer to specific binding and the broken curves to nonspecific binding. ment. One is an increase in ["Hlepinephrine specific binding and the other is a destruction of the a-adrenergic receptors. n addition, the Scatchard analysis showed changes in both Kl, and B, in response to trypsin treatment. This situation makes it difficult to try to answer the important question about how trypsin causes the observed increase in ["Hlepinephrine binding. Therefore, it was decided to use even lower concentrations of trypsin in the treatment and to investigate the possibility that VHepinephrine binding before trypsin treatment may protect against a-receptor destruction. Effects of Treatment of Membranes with Low Concentra- tions of Trypsin before and after Radioligand Binding-Fig. 5 shows the specific binding of 70 nm ["Hlepinephrine and 10 nm ["Hldihydroergocryptine to control membranes and mem-

4 5856 Effects of Trypsin on a-adrenergic Binding branes treated with a low concentration of trypsin (1.5 pg/mg of protein) before radioligand binding (termed pretreatment) or after binding of the radioligands had reached equilibrium (termed post-treatment). is t seen that trypsin post-treatment caused a larger increase in ["Hlepinephrine binding than that obtained with pretreatment. On the other hand, post-treatment caused a smaller decrease in ["Hldihydroergocryptine binding relative to pretreatment. These results are consistent with the possibility that binding of ["Hlepinephrine "protects" the ["Hlepinephrine binding sites against the destructive action of trypsin, suggesting that the effects of the protease may be exerted close to the binding site. The results also suggest that the decrease in ['HHdihydroergocryptine binding is due to trypsin action in the vicinity of ["Hldihydroergocryptine binding sites. Treatment with trypsin before and after addition of propranolol or catechol did not alter the increase in [''Hepinephrine binding. This result suggests that the increase in ["Hlepinephrine binding is not a result of conversion of p- adrenergic to a-adrenergic receptors. Concentration Dependence Curues of r3h]epinephrine Binding to Plasma Membranes Post-treated with Low Con- centrations of Trypsin-Fig. 6 (upper panel) shows the concentration dependence curve of ["Hlepinephrine specific binding to membranes treated after binding with a low concentration of trypsin (1.5 pg/mg of protein) for 10 min. The curve for untreated membranes from Fig. 4 is included for comparison. The lower panel shows the inverted Scatchard plots derived from the concentration dependence curves. Scatchard analysis of the data from the treated membranes yielded an average Ku value of 75 nm and an average B,,, equivalent to 660 fmol/mg of protein. When these values are compared with those obtained for control membranes (Fig. 4), it is seen that in the case of this particular treatment (low concentration of trypsin for a short time after ligand binding), there is no nz W ct W LL \ a cn f 0 5 O 15 [3H] DHYDROERGOCRYPTNE ( nm) 1, BOUND (nm) FG. 7. Effects of trypsin treatment on the concentration dependence curve for specific binding of [3H]dihydroergocryptine. Plasma membranes (approximately 1 mg/ml) were either untreated (0) or treated with 1.5 pg of trypsin/mg of protein (A) for 10 min before ligand binding. The lower panel shows the Scatchard plots derived from the data. Data shown are averages from two experiments (untreated membranes) or from three experiments (treated membranes). 600 n LO 0 UNTREATED -TREATED 1 UNTREATED O0 20 [+] EPNEPHRNE CONCENTRATON (nm) 1 -( \ ,008 0.d12 0.d160.d20 O.d2d (BOUND)/(FREE) FG. 6. Effects of trypsin post-treatment on the concentration dependence curve for specific binding of [3H]epinephrine. Plasma membranes were either untreated (A) or treated with 1.5 pg of trypsin/mg of protein (0) after ligand binding for 10 min. Separation of bound and free ligands was carried out 15 min after the addition of trypsin inhibitor. The lower panel shows the Scatchard plots derived from the data. Data shown are averages from three experiments. FG. 8. Effects of phenoxybenzamine and dihydroergocryptine on trypsin action on [3H]epinephrine specific binding. Liver plasma membranes were either untreated or treated with 1.5 pg of trypsin/rng of protein for 10 min after binding of 50 mm [''Hepinephrine in the presence of 20 nm phenoxybenzamine or 50 nm dihydroergocryptine. Data shown are from one experiment which was repreated once with similar findings. discernible change in affinity and more than a 3-fold increase in the B,,, over that obtained for untreated membranes. Fig. 7 shows the concentration dependence curves for ['H]dihydroergocryptine specific binding to untreated membranes and membranes treated with 1.5 pg of trypsin/mg of protein for 10 min. For untreated membranes, Scatchard analysis of the data yielded a K, of 11 nm and a B,,, equivalent to 1600 fmol/mg of protein. These values are similar to

5 Effects a-adrenergic of Binding on Trypsin 5857 those reported previously (1, 10, ll), whereas for the treated membranes the KO was 14 nm and the BmaX was equivalent to 1100 fmol/mg of protein. t is thus seen that the trypsin treatment caused a decrease in the number of sites which preferentially bind dihydroergocryptine. n view of these data, the possibility of conversion of the ["Hldihydroergocryptine high affinity binding sites to [:'H]epinephrine high affinity binding sites was investigated. Effects of Dihydroergocryptine and Phenoxybenzamine on Trypsin Action on [3H]Epinephrine Binding-Fig. 8 shows the effects of 50 nm dihydroergocryptine and 20 n~ phenoxybenzamine on trypsin action on ["Hlepinephrine binding. The concentrations of dihydroergocryptine and phenoxybenzamine used should block most of the high affinity ["Hldihydroergocryptine sites and have much less effect on ["Hlepinephrine high affinity binding sites (1). t is seen that these concentrations of dihydroergocryptine and phenoxybenzamine caused some inhibition of ["]epinephrine binding to untreated membranes, but greatly depressed the increase in ["Hlepinephrine specific binding caused by trypsin. n addition, it was found that undar these circumstances, trypsin treatment did not cause a significant change in the ability of dihydroergocryptine to inhibit ["Hlepinephrine binding (KW equal to 42 nm for treated membranes compared to 54 nm for untreated membranes). These data indicate that the affinity of dihydroergocryptine for the ['Hlepinephrine high affinity sites was not significantly altered by trypsin treatment. n summary, the results shown in Fig. 8 support the possibility that the VHepinephrine binding increase induced by trypsin is related to the decrease in the [:'H]dihydroergocryptine high affinity binding sites. Time Course of Trypsin Treatment on [3H]Epinephrine and ['HJDihydroergocryptine Binding-Fig. 9 (upperpanel) shows the effects of the duration of trypsin (1.5,ug/mg of 0" 5005, 300- O0 y FG. 9. Time course of trypsin treatment on [3H]epinephrine (EP) and [3H]dihydroergocryptine (DHE) binding. Liver plasma membranes were treated with 1.5 pg of trypsin/mg of protein for the times shown. The trypsin treatments were carried out after binding of 70 nm [,'H]epinephrine had reached equilibrium and before the start of 10 nm [:'H]dihydroergocryptine binding. Measurements are averages offour determinations, and data shown are averages from three experiments. f li\\ E" 500 E z 3 0 z a V" 1 -._""""". [W] DHE P O TRYPSN CONCENTRATON (ug per mg proteml FG. 10. Effects of trypsin concentration on r3h]epinephrine (EPZ) and [3H]dihydroergocryptine (DHE) binding. Liver plasma membranes were treated with increasing concentrations of trypsin for20 min. The trypsin treatments were carried out after binding of 70 nm [jhepinephrine reached equilibrium and before the commencement of 10 nm ['H]dihydroergocryptine binding. Measurements are averages of four determinations and data shown are averages of two experiments. protein) treatment. on ["Hlepinephrine (70 nm) and ["Hldihydroergocryptine (10 nm) specific binding. n the case of ["]epinephrine binding, the trypsin treatment was carried out after binding and then trypsin inhibitor was added at the desired time and binding was assessed 15 min later. n the case of VHdihydroergocryptine binding, the trypsin treatment was carried out before the start of binding. This protocol was used to avoid the postulated protective effects of ["Hdihydroergocryptine on conversion. The lower panel shows the changes in ['Hlepinephrine and ["Hldihydroergocryptine binding as a function of the duration of trypsin treatment. t is seen that there a close is correlation between the gain in ["Hlepinephrine high affinity binding sites and the loss in the ['Hldihydroergocryptine binding sites. This again strongly supports the possibility of interconversion between the two types of binding sites. Effects of Trypsin Concentration on r3hjepinephrine and [3HJDihydroergocryptine Binding-Fig. 10 shows the effects of treatment with various trypsin concentrations on the binding of ["Hlepinephrine (70 nm) and ["Hldihydroergocryptine (10 nm). Plasma membranes were treated with different trypsin concentrations for 20 min. Trypsin treatment was carried out after ["Hlepinephrine binding and before ['Hldihydroergocryptine binding as explained above. t is seen that there is again a good correlation between the gain in ['Hlepinephrine high affinity binding sites and the loss in the ["Hldihydroergocryptine binding sites. This also supports the possibility of interconversion between the two types of binding sites. DSCUSSON Treatment of rat liver plasma membranes with low concentrations of trypsin (1.5 pg/mg of protein) for short periods of time following ["Hlepinephrine binding caused large a increase in binding to the sites which preferentially bind the catecholamine. This increase in the number of binding sites was not accompanied by any change in the dissociation constant. Trypsin treatment also caused a decrease in the number of sites which preferentially bind dihydroergocryptine. The time "_

6 5858 Effects of Trypsin on a-adrenergic Binding course of the increase in the number of binding sites which a continuum of changes in their characteristics. n the first preferentially bind epinephrine was closely correlated with detectable stage, the binding sites have very low affinity for that of the decrease in the binding sites which preferentially bind dihydroergocryptine. A close correlation between these changes was also seen when the concentration of trypsin was changed over a wide range. n addition, trypsin treatment in agonists and bind antagonists preferentially in such a way that the binding cannot be correlated with a-adrenergic effects on physiological processes such as liver glycogenolysis (1). At a second stage, the binding sites behave similarly to what is the presence of a-adrenergic antagonists (dihydroergocryp- expected for the physiological a-adrenergic receptor. They tine and phenoxybenzamine) caused a much less increase in bind agonists and antagonists in a manner which can be ['Hlepinephrine binding than that seen in the absence of these agents. These pieces of evidence are consistent with the proposal correlated with the effects of these agents on glycogen phosphorylase activation and calcium efflux in hepatocytes. At a later stage, the receptors are presumably degraded such that that mild trypsin treatment causes the conversion of the they lose their ability to bind any of the a-adrenergic agents. binding sites which preferentially bind dihydroergocryptine to those which preferentially bind epinephrine. The increase in ["Hlepinephrine binding cannot be attributed easily to the exposure of more binding sites in the plasma membrane as a result of trypsin action because of the following reasons. First, the increase was accompanied by a decrease in ["Hldihydroergocryptine binding. Second, agents which alter Although it is not clear whether these proteolytic effects occur in vivo, it remains possible that proteases could mimic other unknown in vivo changes. Whatever the case may be, the present results support the concept of protease-mediated conversion of the a-adrenergic binding sites which preferentially bind ['Hldihydroergocryptine to the a-adrenergic binding sites which preferentially bind ["Hlepinephrine. membrane structure such as exopeptidases (proteolytic enzymes which act on protein side chains), phospholipases, and Acknowledgments-We would like to thank Mr. Wesley King for various detergents did not cause an increase in [,'H]epinephrine binding. The increase in catecholamine binding also cannot be ashis skillful technical assistance. REFERENCES cribed to the conversion of nonspecific to specific binding 1. El-Refai, M. F., Blackmore, P. F., and Exton, J. H. (1979) J. Biol. sites. This is because the loss of nonspecific sites with trypsin Chem. 254, was very rapid and was not large enough to account for the 2. El-Refai, M. F., and Exton, J. H. (1980) J. Eur. Pharmacol. 62, increase in specific ["Hlepinephrine binding. Conversion of 3. Hanoune, J., Stengel, D., Lacombe, M.-L., Feldmann, G., and P-adrenergic to a-adrenergic sites also seems unlikely since Coudrier, E. (1977) J. Biol. Chem. 252, the number of P-adrenergic sites is too small (60 fmol/mg of 4. Ryan, W., Short, N. A,, and Curtis, G. L. (1975) Proc. Soc. Exp. protein) (10) to account for the increase in a-adrenergic sites. Biol. Med. 150, Furthermore, addition of propranolol prior to trypsin treat- 5. Richert, N. D., and Ryan, R. J. (1977) Proc. Natl. Acad. Sei. U. ment did not alter the increase in ["Hlepinephrine binding S. A. 74, Neurath, H., and Walsh, K. A. (1976) Proc. Natl. Acad. Sei. U. S. induced by the protease. A. 73, The results reported in the present paper suggest another 7. Ribbons, D. W., and Brew, K., eds. Proteolysis and Physiological mechanism by which proteolytic enzymes may play a regula- Regulation, Academic Press, New York tory role in the actions of certain hormones. This postulated 8. Puca, G. A,, Nola, E., Sica, V., and Bresciani, F. (1977) J. Biol. mechanism would involve the existence of physiologically Chem. 252, inactive precursors from which active receptors could be gen- 9. Lowry, 0. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951) J. Biol. Chem. 193, erated by specific and limited proteolysis. These active forms 10. Guellaen, G., Yates-Aggerbeck, M., Vauquelin, G., Strosberg, D.. of the receptor could in turn be degraded by a continuation of and Hanoune, J. (1978) J. Biol. Chem. 253, the proteolytic process. 11. Clarke, W. R., Jones, L. H., and Lefkowitz, R. J. (1978) J. Biol. Trypsin action on the a-adrenergic receptors seems to cause Chem. 253,