Bioavailability of thymic humoral factor gamma2.

Despite the considerable progress made in peptide drug research, many serious obstacles, the most important being that of obtaining sufficient bioavailability for maintaining pharmacological efficiency, still hinder the use of peptides as useful therapeutic agents. Thymic Humoral Factor gamma2 (THF-gamma2) is an immunoregulatory peptide present in thymic extract, identified as an octapeptide of the structure Leu-Glu-Asp-Gly-Pro-Lys-Phe-Leu. THF-gamma2 increases T-cell functions as the response to T-cell lectins and interleukin-2 production. THF-gamma2 has been used as an immunomodulator in clinical conditions associated with immune impairment and dysregulation. The degradation of THF-gamma2 by enzymes present in human plasma, has been investigated. The cleavage of THF-gamma2 was insensitive to classical proteinase inhibitors, but sensitive to metalloproteinase and aminopeptidase inhibitors. The degradation was completely blocked by specific inhibitors of angiotensin converting enzyme (ACE) and aminopeptidase. The sites of cleavage were identified by HPLC analysis and amino acid analysis. Leu1-Glu2 and Lys6-Phe7 bounds were cleaved by aminopeptidase and ACE, respectively. After human ACE purification, some kinetic parameters were determined. Km and Kcat values for THF-gamma2 were 0.273 mM and 107 s-1, respectively. The optimum of pH was 7.6 . I50 for captopril and lisinopril, two specific ACE inhibitors, were 1.052±0.140 nM and 25.770±1.079 nM. The data obtained are useful to modifying the peptide structure so as to reduce recognition by proteinases and to prolong the biological activity.