Biotransformations of pituitary luteinizing hormone in serum and urine. I. Association with serum components.

In the effort to elucidate the nature of luteinizing hormone (LH) in the circulation, studies in adult male rats have been conducted using a highly purified and well-characterized tritiated and methylated ovine pituitary LH, a derivative which retains full biological activity. Following an intravenous injection of the radioactive hormone, serum chromatograms (molecular exclusion chromatography) are characterized by three radioactive components. The first one, a rapidly formed high molecular weight heterogeneous fraction, involves the non-covalent interaction of the hormone with one or more circulating proteins. These high molecular weight complexes are cleared rather slowly from the circulation, relative to the non-associated hormone. A second component co-chromatographs with control hormone and is rapidly cleared from the circulation. These two fractions retain full biological activity judged by their ability to stimulate testosterone production in Leydig cell suspensions. A third component is observed in the circulation about 15 minutes after injection and was shown to represent tritiated amino acids. The lag period for the appearance of the labeled amino acids in the circulation correlates with the kinetics of hepatic and renal uptake and subsequent lysosomal catabolism of [3H]methylated-LH. The high molecular weight fraction may represent a physiologically important circulatory storage form for LH by conferring a relatively long circulatory half-life. This appears to be brought about mainly by reduced urinary excretion, presumably due to its high molecular weight.