Formyl-methyl-methylenetetrahydrofolate synthetase-(combined). An ovine protein with multiple catalytic activities

Abstract

Formyltetrahydrofolate synthetase, methenyltetrahydrofolate cyclohydrolase, and methylenetetrahydrofolate dehydrogenase in sheep liver co-purify 200-fold to yield a homogeneous preparation containing a single protein species observed on discontinuous polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. The synthetase and dehydrogenase activities migrate with the single protein band observed on polyacrylamide gel electrophoresis in two discontinuous buffer systems near pH 8. The protein sediments with a single symmetrical boundary with sedimentation coefficient (S20,w) = 8.30 S and elutes from molecular exclusion chromatography columns at a position corresponding to a diffusion coefficient (D20,w) = 3.99 X 10(-7) cm2 sec-1. Dodecyl sulfate-polyacrylamide gel electrophoresis of the protein intramolecularly cross-linked with dimethylsuberimidate shows one protein species of Mr = 218,000 in addition to the species of Mr = 108,500 characteristic of the unmodified protein. NH2-terminal analysis of the protein using 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) chloride in the presence of dodecyl sulfate results in the recovery of a single dansyl amino acid, alanine. The three activities are thus shown to reside in a protein composed of two apparently identical subunits, and the trivial name formyl-methenyl-methylenetetrahydrofolate synthetase(combined) is proposed for this enzyme to suggest the multiple catalytic activities associated with the single protein species.