Naturally occurring folates in the blood and liver of the rat

Bird, O. D.; McGlohon, Virginia Mims; Vaitkus, John W.

doi:10.1016/0003-2697(65)90138-7

Cited by 147 publications

(37 citation statements)

References 14 publications

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“…This paper suggests that a similar relationship pertains to the carrier-mediated transport of folates in rabbit reticulocytes. In view of this and the observation that the major folate in the blood of man and rodents is 5-methyltetrahydrofolate (19,20), it is likely that uptake of the tetrahydrofolate derivatives of folic acid into folate-dependent tissues is several orders of magnitude greater than that of folic acid itself; thus, folic acid can be of little importance in meeting the requirements of folate-dependent tissues.…”

Section: Mechanism Of Folate Transport In Rabbit Reticulocytesmentioning

confidence: 99%

The mechanism of folate transport in rabbit reticulocytes

Bobzien¹,

Goldman²

1972

J. Clin. Invest.

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A B S T R A C T Folate transport in phenylhydrazine-induced rabbit reticulocytes was studied with the nonmetabolized folate-analog, methotrexate. The time-course of methotrexate uptake into a mixed population of reticulocytes and mature erythrocytes is a two-component process consisting of a small, but rapid, initial uptake phase followed by a much slower uptake component which remains essentially constant over the period of observation. The velocity of the latter uptake component is directly proportional to the per cent reticulocytes and appears to represent a unidirectional influx of methotrexate into these cells. Uptake of methotrexate into reticulocytes was found to have the following characteristics: (a) temperature sensitivity, Qio of 4; (b) uptake velocity as a function of the extracellular methotrexate concentration approximated Michaelis-Menten kinetics with a maximum transport velocity of 48 pmoles/ min per g dry wt; the extracellular methotrexate level at which the uptake velocity was one-half maximum was 1.4 FM; (c) 5-formyltetrahydrofolate markedly inhibited methotrexate uptake but pteroylglutamic acid inhibition was weak; (d) uptake was stimulated in cells preincubated with 5-formyltetrahydrofolate, indicative of heteroexchange diffusion; (e) uptake was independent of extracellular sodium but was inhibited by anions including nitrate, phosphate, and glucose-6-phosphate; (f) uptake was enhanced by azide plus iodoacetate.These data indicate that folate transport in rabbit reticulocytes is mediated by a carrier mechanism which disappears with reticulocyte maturation. The mechanism of folate transport in rabbit reticulocytes is qualitatively similar to tumor cells previously studied; both appear to have an energy-dependent mechanism limiting folate uptake, and influx in both is .inhibited by structurally unrelated inorganic and organic anions. These studies suggest that circulating pteroylglutamic acid is of little importance in meeting the folate requirements of folate-

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Section: Mechanism Of Folate Transport In Rabbit Reticulocytesmentioning

confidence: 99%

The mechanism of folate transport in rabbit reticulocytes

Bobzien¹,

Goldman²

1972

J. Clin. Invest.

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“…In mammals, circulating folates are monoglutamate forms, which are best viewed as the transport forms of this vitamin (12,54). After passage into peripheral cells, folates are converted to poly-␥-glutamate derivatives by the enzyme folylpoly-␥-glutamate synthetase (FPGS) (5,52,60).…”

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A Mouse Gene That Coordinates Epigenetic Controls and Transcriptional Interference To Achieve Tissue-Specific Expression

Racanelli

Turner

Xie

et al. 2008

Molecular and Cellular Biology

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The mouse fpgs gene uses two distantly placed promoters to produce functionally distinct isozymes in a tissue-specific pattern. We queried how the P1 and P2 promoters were differentially controlled. DNA methylation of the CpG-sparse P1 promoter occurred only in tissues not initiating transcription at this site. The P2 promoter, which was embedded in a CpG island, appeared open to transcription in all tissues by several criteria, including lack of DNA methylation, yet was used only in dividing tissues. The patterns of histone modifications over the two promoters were very different: over P1, histone activation marks (acetylated histones H3 and H4 and H3 trimethylated at K4) reflected transcriptional activity and apparently reinforced the effects of hypomethylated CpGs; over P2, these marks were present in tissues whether P2 was active, inactive, or engaged in assembly of futile initiation complexes. Since P1 transcriptional activity coexisted with silencing of P2, we sought the mechanism of this transcriptional interference. We found RNA polymerase II, phosphorylated in a pattern consistent with transcriptional elongation, and only minimal levels of initiation factors over P2 in liver. We concluded that mouse fpgs uses DNA methylation to control tissue-specific expression from a CpG-sparse promoter, which is dominant over a downstream promoter masked by promoter occlusion.

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“…Early studies employed the differential growth requirements of Lactobacillus casei, Streptococcus faecalis, and Pediococcus cerevisiae (3)(4)(5)(6)(7). Subsequently these procedures were improved by prior separation of extracts by ion-exchange column chromatography (8)(9)(10)(11)(12)(13)(14)(15).…”

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“…In the course of this work we have discovered that extraction of folates by using ascorbate solutions at high temperatures leads to chemical interconversions of several of these derivatives. In particular, H4PteGlu gave rise to significant amounts of 5,10-CH2-H4PteGlu and 5-CH3-H4PteGlu, 5,10-CH2-H4PteGlu to 5 Microbiological Assays. Assays were performed as described by Wilson and Home (17), using glycerol-cryprotected L. casei as the inoculum.…”

mentioning

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Evaluation of ascorbic acid in protecting labile folic acid derivatives.

Wilson¹,

Horne²

1983

Proc. Natl. Acad. Sci. U.S.A.

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The use of ascorbic acid as a reducing agent to protect labile, reduced derivatives of folic acid has been evaluated by high-performance liquid chromatographic separations and Lactobacillus casei microbiological assay of eluate fractions. Upon heating for 10 min at 100 degrees C, solutions of tetrahydropteroylglutamic acid (H4PteGlu) in 2% sodium ascorbate gave rise to 5,10-methylene-H4PteGlu and 5-methyl-H4PteGlu. H2PteGlu acid gave rise to 5-methyl-H4PteGlu and PteGlu. 10-Formyl-H4PteGlu gave rise to 5-formyl-H4PteGlu and 10-formyl-PteGlu. 5-Formyl-H4-PteGlu gave rise to a small amount of 10-formyl-PteGlu. 5-Methyl-H4PteGlu and PteGlu appeared stable to these conditions. These interconversions were not seen when solutions of these folate derivatives were kept at 0 degrees C in 1% ascorbate. These observations indicate that elevated temperatures are necessary for the interconversions of folates in ascorbate solutions. Assays of ascorbic acid solutions indicated the presence of formaldehyde (approximately equal to 6 mM). This was confirmed by the identification of 3,5-diacetyl-1,4-dihydrolutidine by UV, visible, and fluorescence spectroscopy and by thin-layer chromatography of chloroform extracts of the reaction mixture of ascorbic acid solutions, acetylacetone, and ammonium acetate. These results indicate that solutions of sodium ascorbate used at elevated temperatures are not suitable for extracting tissue for the subsequent assay of the individual folic acid derivatives.

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Naturally occurring folates in the blood and liver of the rat

Cited by 147 publications

References 14 publications

The mechanism of folate transport in rabbit reticulocytes

The mechanism of folate transport in rabbit reticulocytes

A Mouse Gene That Coordinates Epigenetic Controls and Transcriptional Interference To Achieve Tissue-Specific Expression

Evaluation of ascorbic acid in protecting labile folic acid derivatives.

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