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J. Biol. Chem., Vol. 261, Issue 26, 11949-11955, 09, 1986

Kinetic studies on the processing of human beta-lipotropin by bovine pituitary intermediate lobe pro-opiomelanocortin-converting enzyme

YP Loh

The kinetics of the previously reported paired basic residue-specific pro-opiomelanocortin-converting enzyme from bovine pituitary intermediate lobe secretory vesicles (Loh, Y. P., Parish, D.C., and Tuteja, R. (1985) J. Biol. Chem. 260, 7194-7205) were studied using human 125I-beta-lipotropin as substrate. The enzyme, at a concentration of 20 ng/100 microliters cleaved human beta-lipotropin to yield gamma- lipotropin, a beta-melanotropin linked to beta-endorphin intermediate and beta-endorphin, whereas at an enzyme concentration of 40 ng/100 microliters, the substrate was completely cleaved to yield beta- endorphin and beta-melanotropin. These products were identified by their immunological properties and size on sodium dodecyl sulfate- polyacrylamide gels. The 125I-beta-endorphin product was further shown by high pressure liquid chromatography to contain two forms; the major form co-eluted with 125I-Arg0-beta h-endorphin and the minor form with 125I-beta h-endorphin. No COOH-terminal shortened forms of beta- endorphin were detected. The products formed indicate cleavages at two of the three pairs of basic residues of human beta-lipotropin, at Lys37- Lys38 and Lys57-Arg58, but not at Lys86-Lys87. The cleavage at Lys57- Arg58 occurred primarily in between these basic residues. The Km values for the cleavage of the Lys37-Lys38 and Lys57-Arg58 pairs were 1.9 and 2.5 microM, respectively. The Vmax values for the cleavage of the Lys37- Lys38 and Lys58-Arg58 pairs were 4.8 nmol/micrograms of enzyme/h and 9.1 nmol/micrograms of enzyme/h, respectively.
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