Chemical and enzymatic hydrolysis of dipeptide derivatives of 5-fluorouracil

Abstract

Dipeptides having 5-fluorouracil as an α-substituent on a C-terminal glycine moiety were incubated in buffer solution of pH 7.4 and in calf serum, and the kinetics of their hydrolysis in these media was studied. The stability to chemical and enzymatic hydrolysis depended on the N-terminal amino acid, the configuration of the C-terminal substituted glycine (characterised by direction of optical rotation) and the presence of substituents at the amino and carboxyl end groups of the dipeptides. Thus, the chemical hydrolysis rate decreased with the increase in hydrophobicity of the N-terminal amino acid, and was much lower for dipeptides with free carboxyl end groups than for dipeptides in the ethyl ester form. The highest chemical instability was found for dipeptides with amino and ethyl ester end groups, due to their internal cyclization to the very chemically labile diketopiperazines. Some dipeptides were sensitive to serum amino- and endopeptidases, as the 5-fluorouracil release rate was 1–2 orders of magnitude higher in serum as compared with buffer solutions. The aminopeptidase catalysed hydrolysis rate was higher for dextrorotatory dipeptides and decreased in the order Ala≥Leu>Phe>Gly as N-terminal amino acids. The endopeptidase activity was higher towards levorotatory dipeptides and decreased in the order Ala>Gly>Phe≈Leu at the N-terminus. Presence of free carboxyl instead of ethyl ester end groups at the C-terminus had little influence on aminopeptidase activity, but suppressed the endopeptidase activity.