The phenylalanine and tyrosine content of maternal and fetal body fluids from rabbits fed aspartame

Abstract

The methyl ester of aspartylphenylalanine (aspartame; SC-18862; 3-amino- N-(α-carboxyphenethyl)-succinamic acid, methyl ester) is a sweetening agent which organoleptically has about 180 times the sweetness of sugar. Since this compounds is a food additive its metabolism in pregnancy has been evaluated. Pregnant female rabbits were fed aspartame at a level of 6% in the diet beginning on day 6 of pregnancy. Maternal plasma samples taken at days 6, 9, 16, and 20 were analyzed for phenylalanine and tyrosine. Fetal amniotic fluids were similarly analyzed on days 16 and 20 as were fetal plasma samples at day 20. Maternal plasma phenylalanine and tyrosine increased as a result of the treatment, reaching a peak on day 9. These values returned toward normal levels by day 20. The ratios of fetal/maternal plasma phenylalanine and tyrosine concentrations were unaffected by the treatment. No evidence of substrate inhibition of phenylalanine hydroxylase was seen in maternal or fetal plasma analyses or in in vitro studies of maternal liver homogenates. Tyrosine concentrations in maternal and fetal plasma rose higher with treatment than phenylalanine concentrations. Both amino acids accumulated in the amniotic fluid in amounts greater than, but proportional to, plasma concentrations.