Propagation of allosteric changes through the catalytic-regulatory interface of Escherichia coli aspartate transcarbamylase

Each of two previously isolated strains of Escherichia coli containing a single nonsense codon within the pyrB gene was suppressed with four different nonsense suppressors. The kinetic analysis using crude extracts of these nonsense-suppressed strains indicated that the mutant aspartate transcarbamylases had altered cooperativity and affinity for aspartate as judged by the substrate concentration at half of the maximal velocity. Both pyrB genes were cloned and then sequenced. In both cases, a single base change was identified which converted a glutamine GAC codon into a TAC nonsense codon. Both mutations occurred in the catalytic chain of aspartate transcarbamylase and were identified at positions 108 and 246. The glutamine at position 108 in the wild-type structure is located at the interface between the catalytic and regulatory chains and is involved in a number of interactions with backbone and side chains of the regulatory chain. The glutamine at position 246 in the wild-type structure is located in the 240s loop of the enzyme. Two additional mutant versions of aspartate transcarbamylase were created by site-directed mutagenesis to further investigate the 108-position in the structure, a glutamine to tyrosine substitution at position 108 of the catalytic chain, and an asparagine to glycine change at position 113 of the regulatory chain, a residue which interacts directly with glutamine-108 in the wild-type structure. Both mutant enzymes have reduced affinity for aspartate. However, the Tyr-108 mutant enzyme exhibits a reduced Hill coefficient while the Gly-113 enzyme exhibits an increased Hill coefficient. The response to the allosteric effectors ATP and CTP is also changed for both the mutant enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of various levels of isoflavone aglycone-enriched fermented soybean meal on redox status, serum hormones and milk quality in ewes

The study investigated the effects of various levels of isoflavone aglycone-enriched fermented soybean meal (FSBM) on the redox status, serum hormones and milk quality of ewes from late pregnancy to early lactation. Twenty Chongming White ewes were chosen and divided into four treatment groups (n = 5): basal diet without FSBM (CON); basal diet with 2% (FSBM2); 4% (FSBM4); and 6% FSBM (FSBM6) replacing equal amounts of soybean meal (SBM). At parturition, maternal serum, the placenta and colostrum were collected. At day 21 of lactation, maternal serum and milk were collected. Results showed that, compared with CON, feeding ewes with FSBM6 reduced the concentrations of hydrogen peroxide (H2O2) and 8-hydroxy-2-deoxyguanosine (8-OHdG), and increased the activity of superoxide dismutase (SOD) and total antioxidant capacity (T-AOC) concentration in the placenta. At parturition and day 21 of lactation, the serum SOD activity and T-AOC concentration in FSBM4 and FSBM6 were higher than those in CON. Furthermore, the concentration of serum 8-iso prostaglandin (8-ISO-PGF2α) was markedly lower in FSBM6 than in CON. Serum prolactin (PRL), insulin-like growth factor 1 (IGF-1) and epidermal growth factor (EGF) concentrations were increased in FSBM4 and FSBM6 compared with CON. PRL concentration was increased in FSBM2. FSBM4 increased the levels of fat, protein, lactose, IgA, IgG and IgM in colostrum and milk. In conclusion, feeding ewes with FSBM from late pregnancy to early lactation seemed to increase maternal and placental anti-oxidative capacity, and improve serum hormones and milk quality. Considered overall, the level of 4% supplementation is recommended.Keywords: Milk contents, oxidative stress, placenta, serum parameters