Two methods were used to label insulin receptors covalently with 125I. In the first, an aryl azide derivative of insulin, 125I-labeled 4-azido-2-nitrophenyl-insulin, was synthesized and used to photolabel the binding region of the insulin receptor in rat liver membranes and human placenta membranes. In the second, insulin receptors were purified from rat liver membranes and labeled with 125I by use of chloramine-T; this method presumably has no specificity for the binding region of the receptor. The proteins labeled by both methods were anal zed by sodium dodecyl sulfate/poyacrylamide gel electrophoresis after or without reduction by dithiothreitol. The photoaffinity label specifically labeled a single band in both liver and placenta that had an apparent molecular weight of 135,000 after reduction. A band with similar mobility was present in the chloramine-T-labeled preparation, which also contained a second major band with an apparent molecular weight of 45,000. Without reduction, both methods resulted in a single labeled band with an apparent molecular weight of about 310,000. These results indicate that the insulin receptor of both liver and placenta has a subunit of molecular weight 135,000 that binds insulin and that the receptor may be composed of at least two different subunits that are linked together or greatly stabilized by disulfide bonds. The molecular size of insulin receptors solubilized from several tissues, including membranes from rat liver (1), rat adipocytes (1), human placenta (2, 3), and turkey erythrocytes (4), has been determined by gel filtration. In each tissue the molecular weight has been estimated as being approximately 300,000. This high molecular weight species of the receptor has been reported to dissociate in the presence of insulin into forms of lower molecular weight (3-7). Consistent with this, the major Coomassie blue-staining band seen in sodium dodecyl sulfate (NaDodSO4)/polyacrylamide gels of a highly purified preparation of insulin receptors from liver membranes has an apparent molecular weight of 135,000 (8). A band of similar molecular weight has been specifically labeled in fat cells by using a photoaffinity label (9) or by covalently crosslinking 125I-labeled insulin to its receptor (10).In the present studies the photoaffinity label, 4-azido-2-nitrophenyl-insulin (ANP-insulin), was synthesized and used to label insulin receptors in membranes from rat liver and human placenta. The photoaffinity-labeled bands were compared with the bands present in the purified insulin receptor preparation by NaDodSO4/polyacrylamide gel electrophoresis. METHODSPurification of Insulin Receptor from Rat Liver. Liver membranes were prepared by differential centrifugation as described (11). Insulin receptors were solubilized with Triton X-100 and purified by sequential chromatography on DEAEcellulose, insulin-agarose, and concanavalin A-agarose by methods described previously (8).Iodination of Purified Insulin Receptor. Fifty microliters of 100 mM sodium phosphate buffer (pH 7.4), c...

Iodinated beta H-[2-D-alanine]endorphin exhibits specific binding to cultured human lymphocytes. The binding is inhibited by low concentrations of beta-endorphin and its D-alanine derivative, but is not affected by opiate agonists and antagonists, or by enkephalin analogs, beta-lipotropin, adrenocorticotrophic hormone, or alpha-melanocyte-stimulating hormone; this suggests the existence of a specific, non-opiate binding site (receptor) for beta-endorphin. The carboxy-terminal region of beta-endorphin is essential for this binding activity, since alpha-endorphin is not active. beta-Endorphin may be a circulating hormone with peripheral physiological effects that are not primarily mediated through interactions with opiate or enkephalin receptors.

The synthetic peptide NH2-Tyr-Pro-Phe-Pro-CONH2 (morphiceptin), which is the amide of a fragment of the milk protein beta-casein, has morphinelike activities and is highly specific for morphine (mu) receptors but not for enkephalin (delta) receptors. It is as active as morphine in the guinea pig ileum but much less active in the mouse and rat vas deferens. The discovery of this specific morphine receptor ligand substantiates the hypothesis of multiple opiate receptors. The ligand, which may be of physiological significance since a very similar, or identical, activity can be detected in enzymatic digests of beta-casein, may prove useful for further investigation of the functions of opiate receptor subtypes.

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A bioactive, fluorescent derivative of gonadotropin-releasing hormone,