From adrenocortical tumors. The amount of biological material available as starting material in such a preparation is necessarily limiting and precludes purification of a sufficient amount of receptor needed for the characterization of receptor sequence or for use in reconstitution studies to assess mechanisms for receptor-effector coupling. Regan et al. 6 ; have described the purification of a2-adrenergicreceptors from human platelets, but the immobilized ligand used for affinity chromatography is not available commercially and must be prepared by de m synthesis 7 ; . Furthermore, several chromatographic procedures are required to obtain the same level of purification attained with the present two-step chromatographic protocol. Thus, it is likely that thepresent procedure will be more valuable for the large-scale purification of a2adrenergic receptors for studies of receptor structure and function. All three reported sources of a, -adrenergic receptors identified using covalently-incorporated radioligands demonstrate a comparable apparent molecular size of the receptor on SDSPAGE. [3H]Phenoxybenzamine 6 ; or [3H]SKF-102229 8 ; labeling of affinity-purified human platelet preparations identifies a binding subunitof MI 64, 000. Similarly, also results in the labeling of a broad band with an M, 64, 000 5 ; . The porcine brain a2-adrenergic receptor purified in the present studies migrates 11% SDSon PAGE gels with an M, 65, 000. This apparent molecular weight is not influenced by the addition of a variety of protease inhibitors during the isolation procedures cf. "Materials and Methods" ; , although, as is characteristic of glycoproteins, the apparent M, the receptor is influenced slightly by the degree of of cross-linking and percent acrylamide used in SDS-PAGE. For example, in earlierstudies the lZ5I-rau-AzPCbinding MI species migrated with an apparent 62, 000 when resolved with SDS-PAGE using 10% acrylamide gels 4 ; . The availability of purification procedures for a2-adrenergic receptors derived from several tissue sources will permit the rigorous characterization of the postulates that a2-adrenergic subtypes may exist 9, 10 ; . The accompanying manuscript 17 ; examinesthe modulation of porcine brain a2-adrenergic receptors by Na + , H', and analogs of amiloride that inhibit Na + H exchange andthe retention of these modulatory effects in homogenous preparations of the a2-adrenergic receptor. Acknowledgments-We are grateful to Stephen L. Domino for his of efforts in developing the procedures for synthesis the yohimbine.