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1. Rats were given a purified folate-deficient diet containing 5 g succinylsulphathiazole/kg for 6 5 months in two experiments. Control rats were supplemented with folic acid in the drinking-water.2. Weight gain was much below normal in the folate-deprived rats after the first month. Very low folate levels were recorded in blood, liver and peripheral nerve (12-33% of control). In the central nervous system, including the cerebrospinal fluid, the folate depletion was less conspicuous (50-80% of control). Only marginal signs of anaemia were found and no signs of neurological dysfunction were detected, using nerve conduction velocity measurement and co-ordination tests.3. Light and electron microscopy of the folate deficient liver revealed fatty infiltration, and enlargement of liver parenchymal cells, nuclei and nucleoli. There was often a considerable amount of bile ductular cells in the lobuli but no cirrhosis. The morphological changes resembled those observed in choline deficiency.4. Phospholipid N-methylation in liver was depressed in folate deficiency. This was probably due to a decreased availability of S-adenosylmethionine caused by the low concentrations of methylated folate in liver. Intraperitoneal administration of methionine did not normalize phospholipid methylation.5. In folate deficiency the proportion of ethanolamine phosphoglyceride in liver was increased at the expense of choline phosphoglyceride, which is consistent with a decreased phospholipid methylation. Also an increase in liver triacylglycerol was noted, in accordance with the morphological observations. Brain lipid composition was unchanged.6. After the injection of labelled ethanolamine, isotope accumulated in liver phosphoethanolamine in folate deficiency, probably due to an impairment of the CTP: ethanolaminephosphate cytidylyltransferase (EC2.7.7.14) reaction. The mechanism of this impairment is discussed.7. Although the low concentrations of folate was the main nutritional change in the deprived animals, changes with respect to vitamin B,, and maybe also choline cannot be excluded. We conclude that some of the chsnges in folate deficiency, i.e. fatty liver and decreased biosynthesis of liver phospholipids may be due to a precipitated deficiency of lipotropic agents, whereas other differences may be specific for deficiency of folate per se, such as changes in liver phospholipid fatty acids and some of the morphological aberrations.Folate and vitamin B,, are intimately connected in the methylation of homocysteine to methionine and S-adenosylmethionine (AdoMet), one of the most important methyl-group donors in mammals (Fig.

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