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Nutrition and Aging
Research Communication

Folate Status and Age Affect the Accumulation of L-Isoaspartyl Residues in Rat Liver Proteins^{1 ,2}

Vitamin Metabolism, Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, Boston, MA 02111 and ^* Department of Agricultural Chemistry, College of Agriculture, National Taiwan University, Taipei, Taiwan 10764, Republic of China

³To whom correspondence should be addressed. E-mail: jselhub{at}hnrc.tufts.edu.

Formation of atypical L-isoaspartyl residues in proteins and peptides is a common, spontaneous and nonenzymatic modification of aspartyl and asparaginyl sites. The enzyme protein-L-isoaspartyl methyltransferase (PIMT) catalyzes the transfer of the methyl group of S-adenosyl-L-methionine (SAM) to these L-isoaspartyl sites, thereby allowing reisomerization and restoration of the original alpha peptide linkage. Because SAM is in part a product of folate metabolism, the present study was undertaken to determine the effects of folate deficiency on the presence of L-isoaspartyl residues in hepatic proteins. Young (weanling) and older (12 mo) Sprague-Dawley rats were fed a folate-sufficient (2 mg folate/kg diet) or folate-deficient (0 mg folate/kg diet) diet for 20 wk. Liver proteins were analyzed for L-isoaspartyl residues. This analysis was based on the PIMT-dependent incorporation of [³H]-methyl groups from [³H]-SAM and the subsequent (nonenzymatic) sublimation of these methyl groups into a nonaqueous scintillant. The amount of L-isoaspartyl residues in hepatic proteins was higher in younger folate-deficient than in folate-sufficient rats (deficient: 187 ± 71, sufficient: 64 ± 43 pmol/mg protein, P < 0.025). This difference, however, was not seen among the older groups of rats who instead exhibited a much larger accumulation of L-isoaspartyl residues in their hepatic proteins (deficient: 528 ± 151, sufficient: 470 ± 204 pmol/mg protein, P = 0.568). The importance of these observations is discussed.

KEY WORDS: • folate • L-isoaspartyl • S-adenosyl-L-methionine • protein L-isoaspartyl methyltransferase • rats

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