Kinetic investigation of the functional role of phenylalanine-31 of recombinant human dihydrofolate reductase

The role of the active site residue phenylalanine-31 (Phe31) for recombinant human dihydrofolate reductase (rHDHFR) has been probed by comparing the kinetic behavior of wild-type enzyme (wt) with mutant in which Phe31 is replaced by leucine (F31L rHDHFR). At pH 7.65 the steady-state kcat is almost doubled, but the rate constant for hydride transfer is decreased to less than half that for wt enzyme, as is the rate of the obligatory isomerization of the substrate complex that precedes hydride transfer. Although steady-state measurements indicated that the mutation causes large increases in Km for both substrates, dissociation constants for many complexes are decreased. These apparent paradoxes are due to major mutation-induced decreases in rate constants (koff) for dissociation of folate, dihydrofolate, and tetrahydrofolate from all of their complexes. This results in a mechanism proceeding almost entirely by only one of the two pathways used by wt enzyme. Other consequences of these changes are a much altered dependence of steady-state kcat on pH, inhibition rather than activation by tetrahydrofolate, absence of hysteresis in transient-state kinetics, and a decrease in enzyme efficiency under physiological conditions. The results indicate that there is no quantitative correlation between dihydrofolate binding and the rate of hydride transfer for this enzyme.     

Epidemiology and etiology of diarrhea in colony-born Macaca nemestrina

The epidemiology of diarrhea in colony-born M. nemestrina was studied in 205 neonates and infants in an Infant Primate Research Laboratory (IPRL), and in 248 neonates, juveniles and adolescents up to 4 years of age at a separate breeding and holding facility (Primate Field Station, PFS). Computerized medical records of individual animals over a 5-year period were analyzed to determine the incidence of diarrhea; age, duration and number of episodes; mortality and etiology. The incidence of diarrhea at the IPRL was highest in infants at less than 1 month of age (18.6 cases per 1000 animal days) and at 1-6 months olds (2.0 cases per 1000 animals days). Many infants had multiple episodes. All episodes were less than 10 days in duration. Mortality was low. At the PFS, the highest incidence occurred in infants at 6-12 months of age (1.36 cases per 1000 animal days). Multiple episodes were less common. Duration was variable. The infectious agents diagnosed at both facilities were Shigella, Campylobacter and Cryptosporidium. No pathogens were identified in many episodes. Shigella was more common at PFS than at the IPRL. Chronic diarrhea occurred in approximately 10% of animals at PFS. Intestinal amyloidosis and retroperitoneal fibromatosis were found in 13 animals with chronic diarrhea. Further studies are needed to determine the pathogenesis of chronic diarrhea, the etiologic significance of Campylobacter, and the causes of diarrhea when no pathogens are isolated.