Enhanced utilization of phosphonate and phosphite by Klebsiella aerogenes

Klebsiella aerogenes ATCC 9621 was able to utilize phosphonates (Pn), including aminoethylphosphonate, ethylphosphonate, methylphosphonate (MPn), and phosphonoacetate, and inorganic phosphite (Pt) as sole sources of phosphorus (P). The products of the phn gene cluster were absolutely required for Pn breakdown and Pt oxidation to inorganic phosphate (Pi) in this organism. To determine if K. aerogenes ATCC 9621 could be engineered to enhance the utilization of Pn and Pt, a multicopy plasmid, pBI05, which carried the entire phn gene cluster, was introduced into this strain. Despite the increased dosage of the phn genes, K. aerogenes ATCC 9621(pBI05) could utilize only up to 1.1-fold more Pn and Pt than did the control strain with the parent vector alone. These results suggested that Pi, which was generated from Pn and Pt, might limit further utilization of these P compounds. Consequently, to convert the resulting Pi to polyphosphate (polyP), the plasmid pKP28, which carried the K. aerogenes ppk gene (which encodes polyP kinase), was introduced into K. aerogenes ATCC 9621(pBI05). Overexpression of the ppk gene in K. aerogenes ATCC 9621(pBI05, pKP28) resulted in a 2.5-fold increase in Pt utilization over that of the control strain. This recombinant strain also accumulated approximately sixfold more P than did the control strain when the cells were grown with MPn as a sole source of P.

Metabolism and disposition of dimethyl hydrogen phosphite in rats and mice

A study of dimethyl hydrogen phosphite (DMHP) by the National Toxicology Program (NTP) indicated that chronic administration by oral gavage resulted in an increased incidence of neoplastic lesions in the lungs and forestomachs of Fischer 344 rats but not in B6C3F1 mice. The current study was designed to evaluate the metabolic basis, if any, of this species selectivity by studying the metabolism and disposition of [14C]DMHP in the respective strains of rats and mice. Results of this study indicate that DMHP administered at a range of dose of 10-200 mg/kg was readily and near completely absorbed from the gastrointestinal tracts of rats and mice. DMHP-derived radioactivity was eliminated primarily as CO2 in the expired air, 44-57%, and urine, 28-49%, and very little was collected in feces, 1-2%, or as volatile organics, 2-3%. DMHP-derived radioactivity was widely distributed in tissues of rats and mice, with the highest concentrations observed in the liver, kidneys, spleen, lungs, and forestomach, and the lowest in brain, skeletal muscle, and adipose tissue. The disappearance of radioactivity from mouse tissues was approximately twice as rapid as from rat tissues. In vitro, DMHP was metabolized to formaldehyde by the microsomal fractions of liver, lungs, kidneys, forestomach, and glandular stomach. In vivo, DMHP was metabolized to the product of demethylation, monomethyl hydrogen phosphite (MMHP), which was excreted in urine. Results of this study indicate that the NTP carcinogenicity study with DMHP was carried out within the dose range in which the absorption, metabolism, and disposition of DMHP are linear in both species. Apparent species-dependent differences in the metabolism and disposition of DMHP are limited to the more rapid metabolism and elimination by the mouse. Therefore, the species-dependent variations in the carcinogenicity of DMHP are most likely attributable to factors other than metabolism and disposition.

Activator anion binding site in pyridoxal phosphorylase b: the binding of phosphite, phosphate, and fluorophosphate in the crystal

It has been established that phosphate analogues can activate glycogen phosphorylase reconstituted with pyridoxal in place of the natural cofactor pyridoxal 5'-phosphate (Change YC. McCalmont T, Graves DJ. 1983. Biochemistry 22:4987-4993). Pyridoxal phosphorylase b has been studied by kinetic, ultracentrifugation, and X-ray crystallographic experiments. In solution, the catalytically active species of pyridoxal phosphorylase b adopts a conformation that is more R-state-like than that of native phosphorylase b, but an inactive dimeric species of the enzyme can be stabilized by activator phosphite in combination with the T-state inhibitor glucose. Co-crystals of pyridoxal phosphorylase b complexed with either phosphite, phosphate, or fluorophosphate, the inhibitor glucose, and the weak activator IMP were grown in space group P4(3)2(1)2, with native-like unit cell dimensions, and the structures of the complexes have been refined to give crystallographic R factors of 18.5-19.2%, for data between 8 and 2.4 A resolution. The anions bind tightly at the catalytic site in a similar but not identical position to that occupied by the cofactor 5'-phosphate group in the native enzyme (phosphorus to phosphorus atoms distance = 1.2 A). The structural results show that the structures of the pyridoxal phosphorylase b-anion-glucose-IMP complexes are overall similar to the glucose complex of native T-state phosphorylase b. Structural comparisons suggest that the bound anions, in the position observed in the crystal, might have a structural role for effective catalysis.

The functionalisation of saturated hydrocarbons--XXX. Model studies on the mechanism of some oxygenases

In harmony with our studies on the activation of hydrocarbons by Gif chemistry, we have, in the first part of this paper, studied the mechanism of the lipoxygenase enzymes using soybean lipoxygenase as a model. We have shown with trimethyl phosphite that no free radical is released by the enzyme. In a second part, we have studied the mechanism of the alpha-ketoglutarate dependent enzymes and shown evidence for a mechanism involving the reduction of an intermediate hydroperoxide by the alpha-ketoglutaric acid.