Folate conjugase activity in fresh vegetables and its effect on the determination of free folate content

The levels of naturally-occurring folate conjugates in fresh asparagus, broccoli, Brussels sprouts, cabbage, cauliflower lettuce, and spinach were determined together with the effect on the hydrolysis of pteroylpolyglutamates during the extraction phase of the Lactobacillus casei assay. The levels of conjugate activity vary widely among the vegetables tested, from a low of 0.30 +/- 0.21 in lettuce to a high of 106.2 +/- 6.6 mumoles/hr per 100 mg of fresh weight in spinach. The free folate values (L. casei activity before chicken pancreas conjugase treatment) in the fresh vegetables boiled 1 hr after homogenization were significantly higher than those in vegetables boiled before homogenization, while there were no significant differences in total folate (L. casei activity after treatment with conjugase from chicken pancreas). The increase in free folate values must have been due to the action of endogenous conjugases on the pteroylpolyglutamates from the time the vegetables were homogenized until the conjugase was inactivated by heat. It is concluded that reliable free folate values in fresh vegetables can be obtained only if the naturally present conjugases are inactivated before homogenization of sample in order to prevent the hydrolysis of the pteroylpolyglutamates in the vegetables.