Human sulfite oxidase R160Q: identification of the mutation in a sulfite oxidase-deficient patient and expression and characterization of the mutant enzyme

Sulfite oxidase catalyzes the terminal reaction in the degradation of sulfur amino acids. Genetic deficiency of sulfite oxidase results in neurological abnormalities and often leads to death at an early age. The mutation in the sulfite oxidase gene responsible for sulfite oxidase deficiency in a 5-year-old girl was identified by sequence analysis of cDNA obtained from fibroblast mRNA to be a guanine to adenine transition at nucleotide 479 resulting in the amino acid substitution of Arg-160 to Gln. Recombinant protein containing the R160Q mutation was expressed in Escherichia coli, purified, and characterized. The mutant protein contained its full complement of molybdenum and heme, but exhibited 2% of native activity under standard assay conditions. Absorption spectroscopy of the isolated molybdenum domains of native sulfite oxidase and of the R160Q mutant showed significant differences in the 480- and 350-nm absorption bands, suggestive of altered geometry at the molybdenum center. Kinetic analysis of the R160Q protein showed an increase in Km for sulfite combined with a decrease in kcat resulting in a decrease of nearly 1,000-fold in the apparent second-order rate constant kcat/Km. Kinetic parameters for the in vitro generated R160K mutant were found to be intermediate in value between those of the native protein and the R160Q mutant. Native sulfite oxidase was rapidly inactivated by phenylglyoxal, yielding a modified protein with kinetic parameters mimicking those of the R160Q mutant. It is proposed that Arg-160 attracts the anionic substrate sulfite to the binding site near the molybdenum.     

The interaction of acetoxy-dimethylnitrosamine, a proximate metabolite of the carcinogenic amine, and bacteriophages R17 and T7

The biological and physicochemical effects of reacting bacteriophages R17 and T7 with acetoxy-dimethylnitrosamine (ADMN) have been studied. The rate-determining step in the reactions appeared to be the loss of the acetoxy group by hydrolysis, the hydroxymethyl-methylnitrosamine generated decomposing rapidly to give a methyldiazonium ion and formaldehyde. In experiments with bacteriophage suspended in phosphate buffer the biological inactivation observed was the sum of the effects of the formaldehyde and of alkylation by the methylcarbonium ion produced from the diazonium ion. In experiments with bacteriophage suspended in Tris--HCl buffer the effects of formaldehyde were eliminated by its reaction with the buffer component. Alkylation by the carbonium ion produced unstable phosphotriesters in the bacteriophage RNA which on hydrolysis led to degradation of the molecule. In phosphate buffer the formaldehyde cross-linked the protein coat of the bacteriophage blocking the extraction of the RNA. Estimates of the mean lethal dose and of the extent of degradation of the RNA following reaction in Tris--HCl buffer were fairly close to those observed in experiments with N-methyl-N-nitrosourea (MNUA).     

Variations of glucose, fructose, sorbite and electrolyte concentration following intravenous or intraperitoneal administration of sorbite-electrolyte solution to cattle and sheep

Different quantities of sorbite-electrolyte solution were intravenously administered to eight heads of cattle and four heads of sheep (application values being 50 g sorbite, 0.3049 g MgCl2-6H2O, 0.3728 g KCl, 0.5477 g CaCl2-6H2O, 5.265 g NaCl, 6.804 g sodium acetate-3H2O with 1,000 ml distilled water). Different rises of sorbite, fructose, and glucose were recorded from the blood plasma. Certain manifestations of incompatibility and intolerance phenomena were observed, among them increase of cardiorespiratory activity and muscular tremor. Those findings were obtained primarily from animals which exhibited also strong rise in glucose concentration. One of the sheep died. Larger quantities of solution (2,000 ml or 4,000 ml) were intraperitoneally applied to ten heads of cattle and tolerated by them with no reaction. Sorbite in blood plasma usually reached its maximum two or three hours from application, however, without any rise of fructose or glucose. Slow drip infusion or intraperitoneal infusion are the techniques recommended for application of the above sorbite-electrolyte solution to ruminants.     

Recurrent goiter, hyperthyroidism, galactorrhea and amenorrhea due to a thyrotropin and prolactin-producing pituitary tumor

A 22-year-old woman with recurrent goiter, hyperthyroidism, galactorrhea, and amenorrhea due to a pituitary tumor is described. She had been treated surgically twice for recurrent goiter with tracheal compression. Despite clinical signs of hyperthyroidism and slightly elevated plasma thyroid hormone levels (T4: 11 mug/dl; T3: 189 ng/dl), without thyroid hormone replacement therapy the basal TSH level was elevated up to 23 muU/ml and could not be suppressed by exogenous thyroid hormones: even when the serum thyroid hormone levels were raised into the thyrotoxic range (T4: 16.2 mug/dl T3: 392 ng/dl), the basal TSH fluctuated between 12 and 29 muU/ml. The basal PRL level was elevated up to 6000 muU/ml. The administration of TRH (200 mug iv) led only to small increments of TSH and PRL levels. Bromocriptin (5 mg p.o.) or l-dopa (0.5 g p.o.) suppressed TSH and PRL values significantly. After transsphenoidal hypophysectomy, TSH and PRL were below normal and the patient development panhypopituitarism. The adenoma showed two cell types which could be identified as lactotrophs and thyrotrophs by electronmicroscopy and immunofluorescence. From these data we conclude that the patient had a pituitary tumor with an overproduction of thyrotropin and prolactin.     

Mouse hepatitis virus receptor activities of an MHVR/mph chimera and MHVR mutants lacking N-linked glycosylation of the N-terminal domain

Mouse hepatitis virus binds to the N-terminal domain of its receptor, MHVR, a murine biliary glycoprotein with four immunoglobulin-like domains (G.S. Dveksler, M. N. Pensiero, C. W. Dieffenbach, C. B. Cardellichio, A.A. Basile, P.E. Elia, and K. V. Holmes, Proc. Natl. Acad. Sci. USA 90:1716-1720, 1993). A recombinant protein with only the anchored N-terminal domain was not a functional receptor, but a recombinant protein with the N-terminal domain of MHVR linked to the second and third immunoglobulin-like domains and anchor from the mouse poliovirus receptor homolog, mph, was a functional receptor for mouse hepatitis virus. The native four-domain MHVR has 16 potential N-linked glycosylation sites, including three on the N-terminal domain. Recombinant proteins lacking each one of these three sites or all three of them were functional receptors. Thus, glycosylation of the N-terminal domain is not required, but a glycoprotein longer than the N-terminal domain is required for virus receptor activity.