Tissue-specific expression of functional isoforms of mouse folylpoly-gamma-glutamate synthetase: A basis for targeting folate antimetabolites

Folates and folate antimetabolites are metabolically trapped in mammalian c ells as polyglutamates, a process catalyzed by folylpoly-gamma-glutamate sy nthetase (FPGS). Using 5'-rapid amplification of cDNA ends, RNase protectio n assays, transfection of cDNAs into FPGS-deficient cells, and kinetic anal ysis of recombinant enzymes expressed in insect cells, it was determined th at the species of active FPGS in mouse liver and kidney was different from that in mouse tumor cells, bone marrow, and intestine. The NH2-terminal pep tide of hepatic enzyme contained 18 amino acids not found in enzyme from di viding tissues, and the specificity of the two isoforms for antifolates als o differed, suggesting different architecture of the active sites. In most tissues, the expression of one isozyme or the other was an all-or-nothing e vent. The exclusive use of one of two alternative sets of initial coding ex ons in different tissues underlies this phenomenon, suggesting the design o f antifolates specific for activation by individual FPGS isoforms and hence tissue-selective targeting of antifolate therapy for cancer, arthritis, or psoriasis.