Degradation of thymic humoral factor gamma2 by human plasma: involvement of angiotensin converting enzyme

The degradation of thymic humoral factor-gamma2 (THF-gamma2), an immunoregulatory octapeptide important for T-lymphocyte regulation, by enzymes present in human plasma, was investigated. THF-gamma2 was metabolized through two steps that involved the detaching of N-terminal amino acid leucine followed by hydrolysis of the Lys(6)-Phe(7) bond. The THF-gamma2 cleavages were sensitive to aminopeptidase and metalloproteinase inhibitors. The degradation was completely blocked by amastatin and specific inhibitors of angiotensin converting enzyme (ACE). The cleavages occurred independently, with two different kinetics, faster for the N-terminal hydrolysis than for that of the Lys(6)-Phe(7) bond. Purified human plasma ACE was used to characterize the hydrolysis of Lys(6)-Phe(7) bond. The K(m) and K(cat) values for THF-gamma2 hydrolysis were 0.273 mM and 107 s(-1), respectively. The optimum of chloride concentration was 300 mM, while that of pH was 7.6. The presence of ACE in circulating mononuclear cells raises the possibility that it may play a role in modulating the THF-gamma2 activity.