New aspects of the physiological significance of LRH receptors of pituitary plasma membranes

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Abstract
L-cystine-bis-(4-nitroanilide), a synthetic peptidase substrate, completes with [125I]LRH [luteinizing hormone releasing hormone, luliberin] for specific high affinity LRH receptors on isolated rat pituitary plasma membranes as effectively as a highly active LRH analog [D-ser-(tert-butyl)6-LRH(1-9)-nonapeptide-ethylamide]. The pattern of [125I]LRH degradation by isolated pituitary plasma membranes was similar to that effected by pituitary and hypothalamic enzyme extracts. The enzyme activity seemed to accumulate in the supernatant of the membrane suspension after being released from the membrane structure. Plasma membrane supernatant and pituitary and hypothalamic enzyme extracts were chromatographed together with labeled LRH on Sephadex G 200. This resulted in protein binding patterns with 1 peak at the fractions representing a MW of 600,000 to 650,000 and another one of approximately 80,000, the 1st probably being an aggregate of the latter. After solubilization of the plasma membranes by Triton X-100, there was no evidence for another LRH receptor. There were also 2 peaks of [125I]LRH degrading activity (with identical metabolic pattern) and 2 peaks of arylamidase activity which were, however, not identical with the binding peaks. Both the arylamidase and the LRH binding-pattern were demonstrated to be temperature- and hormone-dependent and seemed to be functionally related. The results indicate that there is a functional relationship between LRH binding-sites on plasma membranes and LRH degrading enzymes. This implies that the search for the true LRH receptor has to be directed towards intracellular compartments of the pituitary gonadotrophs.