Microbiological assay is still widely used for estimating folic acid derivatives in serum and other biological samples. We describe here a modification of this procedure involving use of 96-well microtiter plates. This procedure, used with modern, computer-interfaced microtiter-plate readers and data-reduction software, greatly shortens the time and minimizes reagent costs for this assay. Under the conditions of our assay procedures, all folic acid derivatives tested gave equal growth response for Lactobacillus casei. Results for assays of rat liver extracts showed excellent agreement between the standard bioassay and the 96-well procedure.

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Effects of aging on hepatic folate metabolism and transport were assessed in male Fisher 344 rats. Total serum and hepatic folate levels were measured. Hepatic folates were measured by high-performance liquid chromatography and by Lactobacillus casei assay. Transport of 5-methyltetrahydrofolate (5-CH3-H4PteGlu) was measured in isolated hepatocytes. Serum folate declined with aging; however, neither the total folate level nor the distribution of hepatic folate coenzymes was affected by the aging process. The level of liver folate monoglutamates was not significantly different in any group. The initial rate of uptake of 5-CH3-H4PteGlu was significantly decreased in hepatocytes from the 24-mo-old rats, as was the ability to concentrate this folate from the medium. Aged rats maintain apparently normal levels of hepatic folates despite decreased serum levels and decreased ability to take up folates, suggesting that membrane transport of folates may not be a limiting factor in hepatic folate assimilation.

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