No abstract

Glycine N-methyltransferase (GNMT) is inhibited by 5-methyltetrahydrofolate polyglutamate in vitro. It is believed to play a regulatory role in the synthesis de novo of methyl groups. We have used the amino-acid-defined diet of Walzem and Clifford [(1988) J. Nutr. 118, 1089-1096] to determine whether folate deficiency in vivo would affect GNMT activity, as predicted by the studies in vitro. Weanling male rats were fed on the folate-deficient diet or a folate-supplemented diet pair-fed to the deficient group. A third group was fed on the folate-supplemented diet ad libitum. Development of folate deficiency rapidly resulted in decreased levels of S-adenosylmethionine (SAM) and elevation of S-adenosylhomocysteine (SAH). The ratios of SAM to SAH were 1.8, 2.7 and 1.5 in the deficient group for weeks 2, 3 and 4 of the experiment, and the values were 9.7, 7.1 and 8.9 for the pair-fed control group and 10.3, 8.8 and 8.0 for the control group ad libitum fed. The activity of GNMT was significantly higher in the deficient group than in either of the two control groups at each time period. This was not due to increased amounts of GNMT protein, but reflected an increase in specific enzyme activity. Levels of folate in both the cytosol and mitochondria were severely lowered after only 2 weeks on the diet. The distribution of folate coenzymes was also affected by the deficiency, which resulted in a marked increase in the percentage of tetrahydrofolate polyglutamates in both cytosol and mitochondria and a very large decrease in cytosolic 5-methyltetrahydrofolate. The increased GNMT activity is therefore consistent with decreased folate levels and decreased inhibition of enzyme activity.

No abstract

Several studies have suggested that the metabolism of one-carbon compounds may have a special role in the function of the exocrine pancreas. An amino acid-defined diet was used to produce folate deficiency in a group of male rats. These rats were compared with a group of rats pair-fed the same diet supplemented with adequate folate and with a third group fed the folate-supplemented diet with ad libitum access. Pancreatic folate concentrations were already severely depleted after 4 wk of feeding the deficient diet (0.95 +/- 0.10, 5.81 +/- 0.29 and 4.58 +/- 0.30 nmol/g for the deficient, pair-fed control and ad libitum-fed control groups, respectively). The level of folate present in the pancreas of nondeficient animals was second only to that reported for liver. Urinary amylase excretion by animals in the deficient group was higher than that by the other groups (245.5 +/- 21.9, compared with 181.9 +/- 14.5 and 195.3 +/- 10.9 units/mg creatinine for the deficient, pair-fed control and ad libitum-fed control groups, respectively) after 4 wk. The ratio of S-adenosylmethionine to S-adenosylhomocysteine was 18.6 +/- 1.6 and 14.5 +/- 1.0 after 4 wk for the ad libitum-fed control and pair-fed control groups, respectively, but was significantly lower at 6.3 +/- 1.1 for the deficient group. These results indicate a profound effect of folate deficiency upon methyl group metabolism of the pancreas and suggest that this may result in decreased pancreatic function.

Previous studies have suggested that the metabolism of methyl groups is an important factor in the function of the exocrine pancreas. Ethionine, an inhibitor of cellular methylation reactions, produces hemorrhagic pancreatitis when administered to mice fed a choline-deficient diet. Glycine N-methyltransferase, an enzyme which regulates the ratio of S-adenosylmethionine to S-adenosylhomocysteine, is particularly abundant in the exocrine pancreas. Since de novo synthesis of methyl groups requires the participation of folate coenzymes, we investigated the effect of folate deficiency on pancreatic exocrine function. Rats were fed an amino acid-defined folate-deficient diet or the same diet supplemented with folate ad libitum. A third group received the folate supplemented diet pair-fed to the deficient group. After 3 and 5 wk, pancreatic amylase secretion was measured in perfused duodenal segments of anesthetized animals before and after cholecystokinin injection. Pancreatic secretion was significantly reduced in the deficient group compared with the pair-fed control group after 5 wk. These results indicate that severe folate deficiency impairs pancreatic exocrine function.