Breakdown and fate of ACTH and MSH

Abstract

Exogenously administered ACTH and MSH have short circulatory half-lives, in the order of minutes. When pituitary ACTH secretion is abolished or suppressed by hypoplysectomy or treatment with corticosteroids, there is also a rapid decline of endogenous ACTH from plasma. Inactivation of ACTH in blood is too slow to account for the rapid inactivation in vivo indicating that ACTH degradation takes place in one or more tissue compartments rather than in plasma itself. Experiments on the tissue distribution of radioactive ACTH indicate that the kidney and to a lesser extent liver, muscle and adipose tissue are principal sites of ACTH penetration. The biphasic decay rates after injection of ACTH consist of a fast component, representing equilibration between plasma and various tissue spaces, and a slower component, reflecting subsequent metabolism in the latter compartments. In general, metabolism of ACTH within tissues has been studied by incubating tissue samples (fragments, suspensions, homogenates, or subcellular fractions) with ACTH and ACTH-like peptides. In many of these studies endopeptidases have been found to act early in the degradation process. Detailed studies on ACTH metabolism in rat intestine indicate that Ph 7-Arg 8 is the first bond to break followed by cleavages at the surrounding sites to release free amino acids. ACTH degrading enzymes in adrenal tissue seem to have a predominantly tryptic specificity. The finding of ACTH-(18–39), or corticotrophin-like intermediate lobe peptide (CLIP), in the pars intermedia of pituitary, suggests cleavage of ACTH in the 13–18 region. However, pituitary enzymes with this specificity, and enzymes amidating and acetylating ACTH-(1–13) to yield α-MSH have not yet been demonstrated. In mouse brain preparations ACTH (1–24) is rapidly degraded to free amino acids. The bulk of the degrading activity resides in the cytosol and seems to be of a nonlysosomal origin. The data obtained suggest that the initial rate-limiting steps in breakdown consist of internal bond cleavages followed by rapid breakdown of the resulting smaller fragments. In a series of ACTH analogs rate of degradation was found to increase with decreasing fragment size. In agreement with these findings the action of aminopeptidase M on various ACTH fragments was also found to be size-dependent. This, and the reported inverse relationship between the size of β-endorphin fragments and their degredation rate, suggest that the relationship between breakdown and fragment size may apply in general to the degradation of biologically active peptides. Some of the enzymes acting on the N-terminal region of ACTH have been identified. For example, the breakdown of ACTH-(1–4) by brain supernatant can be accounted for by the presence of the two enzymes, a neutral arylamidase, also implicated in the degradation of enkephalins, and a -SH inhibited dipeptidase. It is likely that common mechanisms underly the degradation of many peptides in brain and other tissues. For instance, angiotensin converting enzyme of brain can attack bradykinin and enkephalins as well as angiotensin; neutral arylamidase of brain cleaves the N-terminus of a variety of peptides, including ACTH, enkephalins and MIF; postproline cleaving enzyme of brain can deamidate TRH and cleave LH-RH. Although the overall framework of ACTH degradation is beginning to emerge, detailed knowledge of individual enzymatic steps is scattered and fragmentary. It is clear that this information is necessary to understand the hormonal and behavioral effects of ACTH. In addition questions as to the biosynthesis and metabolism of specific fragments can only be understood when the action of degrading enzymes is more completely defined. Such knowledge would enable the design of structurally modified peptides with greater resistance to degradation and enhanced biological activity. Work in our institute as well as in other laboratories is currently aimed at isolating, purifying and characterizing these hydrolases, in particular the endopeptidases which play such an important role in the initial steps of hormone metabolism.