Structural requirements for the activity of antifolates as substrates for mammalian folylpolyglutamate synthetase.

The activity of mouse-liver folylpolyglutamate synthetase (FPGS) was compared using a number of folates and folate analogs in order to determine which structure modifications were compatible with enzyme catalysis and with efficient binding to enzyme. Most structural alterations in the amino acid side chain eliminated activity as a substrate for this enzyme, whereas modifications of any of several positions in the pteridine ring were tolerated with retention of FPGS substrate activity. Folate analogs with the lowest apparent Michaelis constants (Km,app) had a) a 4-amino group, b) a 5,6,7,8 reduced ring system, c) a quinazolate ring, and/or d) an unsubstituted 10-position. There was some correlation between FPGS substrate activity and the potency of folate antimetabolites as cytotoxic compounds but not necessarily as compounds selectively cytotoxic to tumor cells.