REGULATION OF FOLATE AND ONE-CARBON METABOLISM IN MAMMALIAN-CELLS .2.EFFECT OF FOLYLPOLY-GAMMA-GLUTAMATE SYNTHETASE SUBSTRATE-SPECIFICITY AND LEVEL ON FOLATE METABOLISM AND FOLYLPOLY-GAMMA-GLUTAMATE SPECIFICITY OF METABOLIC CYCLES OF ONE-CARBON METABOLISM
Ke. Lowe et al., REGULATION OF FOLATE AND ONE-CARBON METABOLISM IN MAMMALIAN-CELLS .2.EFFECT OF FOLYLPOLY-GAMMA-GLUTAMATE SYNTHETASE SUBSTRATE-SPECIFICITY AND LEVEL ON FOLATE METABOLISM AND FOLYLPOLY-GAMMA-GLUTAMATE SPECIFICITY OF METABOLIC CYCLES OF ONE-CARBON METABOLISM, The Journal of biological chemistry, 268(29), 1993, pp. 21665-21673
The effect of folylpoly-gamma-glutamate synthetase (FPGS) levels on fo
late accumulation was investigated in Chinese hamster ovary cells expr
essing various levels of human and Escherichia coli FPGS activity. At
low medium folate concentrations, folate accumulation was limited by i
nflux and was independent of FPGS activity except in cells expressing
extremely low levels of FPGS. Essentially all transported folate was m
etabolized to retained polyglutamate derivatives, the chain length of
which varied with the level of FPGS activity. As medium folate concent
ration increased through the physiological to the pharmacological rang
e, cellular folate accumulation became proportional to FPGS activity a
nd the chain length of intracellular folates decreased. At high folate
concentrations, competition between substrates for FPGS limited the e
xtent of polyglutamylation and less than 5% of transported folate was
retained by the cell. Pteroyltriglutainates functioned as effectively
as the longer chain length polyglutamates normally found in mammalian
cells in the metabolic cycles of de novo purine and thymidylate biosyn
thesis but were unable to support glycine and methionine synthesis. Tr
ansfectants expressing human FPGS and containing folates of glutamate
chain length ranging from four to eight were equally effective at supp
orting glycine synthesis, and transfectants expressing higher levels o
f FPGS were able to grow in the absence of methionine. Growth in the a
bsence of methionine required high (nonphysiological) intracellular fo
late levels and longer chain length polyglutamates.