The effect of maternal amino acid imbalance on fetal cerebral polyribosomes

Experimental hyperphenylalaninemia in the pregnant guinea pig: possible phenylalanine teratogenesis and p-chlorophenylalanine embryotoxicity

Maternal hyperphenylalaninemia (HPH) due to deficient phenylalanine (Phe) hydroxylation is a recognized human teratogen associated with an increased incidence of intrauterine growth retardation, microcephaly, congenital heart disease, and mental retardation. There are no previous reports of experimental HPH during organogenesis. Sustained HPH was produced in pregnant guinea pigs by adding 3.5% Phe and 1.0% parachlorophenylalanine (pCPA), an inhibitor of Phe hydroxylase, to standard guinea pig chow. Animals consumed the supplemented test diets from gestation day 1 until killed on gestation day 17. Examination of day 17 embryos revealed that embryonic mortality was associated only with maternal pCPA administration and was independent of the degree of maternal HPH. Embryonic malformation was associated with maternal HPH as well as maternal pCPA administration. Both maternal HPH and pCPA administration were associated with embryonic growth retardation. There was no association between maternal food intake or plasma tyrosine levels and embryonic abnormality or mortality. Both Phe and tyrosine were found to be concentrated in gestation day 17 yolk sac fluid when compared to maternal plasma Phe and tyrosine. The association of embryonic malformation and maternal HPH is consistent with human data. The embryotoxicity of pCPA requires further study and highlights the necessity of appropriate controls in models of experimental HPH.

Effects of phenylalanine loading on protein synthesis in the fetal heart and brain of rat: an experimental approach to maternal phenylketonuria

Pregnant rats were loaded with L-phenylalanine, and the distributions of [14C]leucine and [14C]urea into fetal plasma and tissues were examined. Uptake of [14C]leucine into the supernatant and protein fractions of fetal plasma and tissues was low in the rats loaded with phenylalanine. In contrast, [14C]urea was distributed identically in both groups, indicating that maternal hyperphenylalaninemia did not affect blood flow across the placenta. Administration of phenylalanine and p-chlorophenylalanine produced amino acid imbalance in fetal tissues. Along with these changes, polysomes of the affected fetal heart and brain disaggregated without changes in the ribonuclease activity. These results indicate that high phenylalanine levels in maternal plasma disturb the active transport of amino acids across the placenta, causing an amino acid imbalance and disaggregation of polysomes in fetal heart and brain. These changes may contribute to the congenital heart disease and mental retardation of maternal phenylketonuria.

Cerebral ribosomal protein phosphorylation in experimental hyperphenylalaninaemia

Investigations were carried out on the effects of phenylalanine loading on ribosomal protein phosphorylation in cerebral cortices of infant rats. Administration of L-phenylalanine intraperitoneally, in doses of 1 or 2 mg/g body wt., resulted within 30 min in a significant decrease in incorporation of radioactivity from intracisternally administered [32P]Pi into constitutive ribosomal proteins of the cerebral 40S subunit. This phenomenon was not accompanied by significant variations in 32P uptake into the cerebral cytosol. Incorporation of radioactivity into ribosomal proteins of the cerebral 60S subunit exhibited only minor variations under these circumstances. Alterations in the phosphorylation state of cerebral 40S ribosomal proteins induced by phenylalanine loading involved principally the S6 protein, which exists in multiple states of phosphorylation. The proportions of the more highly phosphorylated congeners of this protein were markedly decreased, as detected by two-dimensional electrophoretograms and autoradiographs of the cerebral 40S ribosomal proteins. Phenylalanine loading also altered the relative extent of phosphorylation of the S6 protein in cerebral polyribosomes and monoribosomes. In control animals, the specific radioactivity of 40S proteins in cerebral polyribosomes was five to ten times that of 40S proteins in the monoribosome population. At 1 h after phenylalanine administration, the specific radioactivities of 40S proteins in the two ribosome populations tended to approach equality. These alterations in ribosomal protein phosphorylation were accompanied by a decrease in the proportion of polyribosomes in purified ribosome preparations isolated from cerebral cortices of phenylalanine-treated infant rats. In animals given the higher dose of phenylalanine (2 mg/g body wt.), subsequent administration of a mixture of seven neutral amino acids, which resulted in partial recovery of polyribosomes, also tended to reverse the changes in ribosomal protein phosphorylation. Variations in the activities of ribonuclease enzymes in the cerebral cytosol were also observed under these conditions. Administration of phenylalanine increased the activities of cerebral ribonucleases, whereas subsequent treatment with the amino acid mixture partly reversed this effect. The results suggest that alterations in cerebral ribosomal protein phosphorylation, ribosome aggregation and ribosome function are interrelated in experimental hyperphenylalaninaemia.