Description: Fetal growth restriction (FGR) is the inability of a fetus to reach its genetically predetermined growth potential. In the absence of a genetic anomaly or maternal undernutrition, FGR is attributable to "placental insufficiency": inappropriate maternal/fetal blood flow, reduced nutrient transport or morphological abnormalities of the placenta (e.g., altered barrier thickness). It is not known whether these diverse factors act singly, or in combination, having additive effects that may lead to greater FGR severity. We suggest that multiplicity of such dysfunction might underlie the diverse FGR phenotypes seen in humans. Pregnant endothelial nitric oxide synthase knockout (eNOS –/– ) dams exhibit dysregulated vascular adaptations to pregnancy, and eNOS –/– fetuses of such dams display FGR. We investigated the hypothesis that both altered vascular function and placental nutrient transport contribute to the FGR phenotype. eNOS –/– dams were hypertensive prior to and during pregnancy and at embryonic day (E) 18.5 were proteinuric. Isolated uterine artery constriction was significantly increased, and endothelium-dependent relaxation significantly reduced, compared with wild-type (WT) mice. eNOS –/– fetal weight and abdominal circumference were significantly reduced compared with WT. Unidirectional maternofetal 14 C-methylaminoisobutyric acid (MeAIB) clearance and sodium-dependent 14 C-MeAIB uptake into mouse placental vesicles were both significantly lower in eNOS –/– fetuses, indicating diminished placental nutrient transport. eNOS –/– mouse placentas demonstrated increased hypoxia at E17.5, with elevated superoxide compared with WT. We propose that aberrant uterine artery reactivity in eNOS –/– mice promotes placental hypoxia with free radical formation, reducing placental nutrient transport capacity and fetal growth. We further postulate that this mouse model demonstrates "uteroplacental hypoxia," providing a new framework for understanding the etiology of FGR in human pregnancy.

Description: Glycine decarboxylase, or P-protein, is a pyridoxal 5′-phosphate (PLP)-dependent enzyme in one-carbon metabolism of all organisms, in the glycine and serine catabolism of vertebrates, and in the photorespiratory pathway of oxygenic phototrophs. P-protein from the cyanobacterium Synechocystis sp. PCC 6803 is an α2 homodimer with high homology to eukaryotic P-proteins. The crystal structure of the apoenzyme shows the C terminus locked in a closed conformation by a disulfide bond between Cys972 in the C terminus and Cys353 located in the active site. The presence of the disulfide bridge isolates the active site from solvent and hinders the binding of PLP and glycine in the active site. Variants produced by substitution of Cys972 and Cys353 by Ser using site-directed mutagenesis have distinctly lower specific activities, supporting the crucial role of these highly conserved redox-sensitive amino acid residues for P-protein activity. Reduction of the 353–972 disulfide releases the C terminus and allows access to the active site. PLP and the substrate glycine bind in the active site of this reduced enzyme and appear to cause further conformational changes involving a flexible surface loop. The observation of the disulfide bond that acts to stabilize the closed form suggests a molecular mechanism for the redox-dependent activation of glycine decarboxylase observed earlier.