Carboxypeptidase displaying differential velocity in hydrolysis of methotrexate, 5-methyltetrahydrofolic acid, and leucovorin

An enzyme that catalyzes the hydrolysis of folic acid and the antifolate methotrexate nearly 20 times more rapidly than the hydrolysis of 5-methyltetrahydrofolate was extraced from a gram-negative bacterium tentatively identified as a Flavobacterium sp. The enzyme was purified 500-fold and found to have a molecular weight of about 53,000. Apparently a metallo-enzyme, it is inhibited by citrate and ethylenediaminetetraacetic acid (EDTA). Ca2+, Co2+, Mg2+, and Zn2+ reverse inhibition by EDTA, whereas Ca2+ and Zn2+ are weak activators in the absence of EDTA. The enzymatic reaction releases the carboxy-terminal glutamyl moiety of derivatives of pteroyl-mono-L-glutamic acid. Substituents on N5 of the pteridine ring decrease the velocity of hydrolysis. Some non-specificity for the terminal amino acid is expressed. The strikingly different rates of hydrolysis of methotrexate and 5-methyltetrahydrofolate have stimulated interest in this enzyme for its potential clinical value in improving the therapeutic index of methotrexate.