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Rat Liver Subcellular Folate Distribution Shows Association of Formyltetrahydropteroylpentaglutamates with Mitochondria and Methyltetrahydropteroylhexaglutamates with Cytoplasm^1,2,

G. Franklin Carl^*,,3, Farlyn Z. Hudson^* and Byron S. McGuire, Jr.^*

^* Medical Research Service, Veterans Affairs Medical Center, Augusta, GA 30904 Department of Neurology, Medical College of Georgia, Augusta, GA 30912

The ternary complex method for the determination of folylpolyglutamates was combined with procedures for interconverting folate derivatives to measure 28 different folate derivatives in the subcellular fractions of rat liver. Folates in the homogenate showed a typical distribution with nearly equal quantities of penta- and hexaglutamates and pteridine derivatives in decreasing order as follows: 1) methyl substituted folates [5-methyltetrahydropteroylglutamates], 2) unsubstituted folates [tetrahydropteroylglutamates + 5,10-methylenetetrahydropteroylglutamates], 3) formyl substituted folates [5-formyltetrahydropteroylglutamates + 10-formyltetrahydropteroylglutamates + 5,10-methenyltetrahydropteroylglutamates], and 4) oxidized folates [dihydropteroylglutamates]. In the homogenate the methyl substituted folates exhibited a higher hexa:pentaglutamate ratio than did the other pteridine derivatives. As the fractionation proceeded toward purer subcellular components, the methyl substituted folates were found almost exclusively in the soluble fraction, and this fraction also contained the higher hexa:pentaglutamate ratio characteristic of the methyl substituted folates. The plasma membrane, the microsomal and the nuclear fractions did not contain appreciable folate. The mitochondrial fraction contained primarily formyl substituted and unsubstituted folates, and these folates exhibited the lower hexa:pentaglutamate ratios. These data support the hypothesis that folate-dependent one-carbon metabolism is compartmentalized in the eukaryotic cell.

KEY WORDS: • mitochondria • cytoplasm • rats • pteroylpolyglutamates • liver

¹ Supported by the Medical Research Service of the Department of Veterans Affairs.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 5 December 1994. Revision accepted 27 February 1995.