Poster: Secondary metabolism

Abs # 256: Leucovorin and one-carbon (C₁) metabolism in Arabidopsis

Presenter:	Collakova, Eva , collakov@msu.edu
Authors	Collakova, Eva  (A)   Goyer, Aymeric J (B)   Hanson, Andrew D (B)   Shachar-Hill, Yair  (A)
Affiliations:	(A): Michigan State University (B): University of Florida

Tetrahydrofolates (THF) are essential donors of C₁ units in vital reactions in all organisms. N⁵-formyl THF, also known as leucovorin, is a major THF in plant mitochondria. In spite of its abundance, relatively little is known about the origin and roles of this stable form of THF in plant C₁ metabolism. Leucovorin inhibits serine hydroxymethyltransferase (SHMT) at physiological concentrations and is probably formed from N^5,10-methenyl THF in the presence of glycine as a side product of SHMT [Stover & Schirch (1990). JBC 265: 14227]. Therefore, increased production of glycine during photorespiration may lead to the elevated synthesis of leucovorin by SHMT and the subsequent inhibition of this enzyme in mitochondria. To avoid potential inhibition of photorespiration, leucovorin levels may be regulated by N⁵-formyl THF cycloligase, a mitochondrial enzyme that converts leucovorin to N^5,10-methenyl THF [Roje et al. (2002). JBC 277: 42748]. Although the formyl group of leucovorin can enter C₁ metabolism in this manner, its major role is thought to be the regulation of C₁ transfers [Stover & Schirch (1993). TIBS 18: 102]. To further investigate the roles of leucovorin in these processes, wild type and N⁵-formyl THF cycloligase knockout mutant Arabidopsis plants were studied. Preliminary ¹H NMR fingerprinting experiments demonstrated that the mutant plants had elevated levels of glycine, consistent with SHMT inhibition by elevated leucovorin levels. Labeling experiments using wild type and mutant plants and various ¹³C₁ labeled donors are currently underway to follow fluxes through serine and glycine pathways.