Folic acid metabolism in vitamin B12-deficient sheep. Depletion of liver folates

Abstract

1. Metabolism of folate was studied in six ewes in an advanced state of vitamin B12 deficiency as judged by voluntary food intake and in their pair-fed controls receiving vitamin B12. A group of four animals that were maintained throughout the experiment at pasture was also studied. 2. After 34–40 weeks on the cobalt-deficient diet urinary excretion of formiminoglutamate by four deficient animals was about 3.2mmol/day and this was not significantly decreased by injection of three of them with about 4.5μg of [2-14C]folate/kg body weight per day for 5 days. Three days after the last injection retention of [2-14C]folate by the livers of the deficient animals (5.5% of the dose) was lower than that of their pair-fed controls (26% of the dose) but there was no evidence of net retention of injected folate in the livers of either group. Urinary excretion of 14C indicated that renal clearance of folate may have been impaired in very severe vitamin B12 deficiency. 3. As estimated by microbiological assays total folates in the livers of animals at pasture (12.9μg/g) included about 24% of 5-methyltetrahydrofolate as compared with about 72% of a total of 12.5μg/g in three further ewes fed on a stock diet of wheaten hay-chaff and lucerne-chaff. Liver folates of vitamin B12-deficient animals (0.5μg/g) included about 88% of 5-methyltetrahydrofolate as compared with about 51% of a total of 5.2μg/g in pair-fed animals treated with vitamin B12. 4. Chromatography of liver folates of the pair-fed animals permitted quantitative estimates of the pteroylglutamates present. The results showed that the vitamin B12-deficient livers were more severely depleted of tetrahydrofolates and formyltetrahydrofolates than of methyltetrahydrofolates and that as the deficiency developed they were more severely depleted of the higher polyglutamates than of the monoglutamate within each of these classes. Results from animals injected with [2-14C]folate indicated an impairment of the exchange between pteroylmonoglutamates and pteroylpolyglutamates in the livers of deficient animals. 5. In vitamin B12-deficient animals with food intakes below 200g/day some of the liver folates were not completely reduced and some degradation of pteroylpolyglutamates was detected. The latter condition may have been associated with fatty liver. 6. The results are discussed in relation to current theories of vitamin B12–folate interactions.