Brain pyridoxine-5-phosphate oxidase. Modulation of its catalytic activity by reaction with pyridoxal 5-phosphate and analogs

Pyridoxine-5-P oxidase, the flavoprotein involved in the oxidation of pyridoxamine-5-P and pyridoxine-5-P to pyridoxal-5-P, has been isolated and purified to homogeneity using sheep brain tissues. Inactivation of the oxidase by bis-pyridoxal-5-P results in binding of the inhibitor to specific lysyl residues. After NaBH4 reduction of the inactivated enzyme, it was found that 1 P-pyridoxyl-pyridoxine-P residue was incorporated per enzyme dimer. After trypsin digestion of the bis-PLP modified enzyme, only one peptide absorbing at 320 nm, was separated by reverse-phase high performance liquid chromatography. The amino acid sequence of the labeled peptide was determined by automated Edman degradation. The observations reported in this paper are relevant to the mechanisms underlying the regulation of the catalytic function of pyridoxines-5-P oxidase by the product pyridoxal-5-P. It is postulated that the catalytic function of the oxidase is modulated by binding of pyridoxal-5-P to a specific lysyl residue of the dimeric structure of the protein.

Biosynthesis of 4-aminobutyrate aminotransferase

Mitochondrial 4-aminobutyrate aminotransferase was synthesized in a cell-free reticulocyte lysate using polysomal RNA isolated from pig brain. Its primary translation product has a higher molecular mass than the mature enzyme. The difference in relative molecular mass is approximately 2000 as revealed by SDS/polyacrylamide gel electrophoresis. The precursor of 4-aminobutyrate aminotransferase recognizes polyvalent antibodies raised against the mature enzyme. The precursor of 4-aminobutyrate aminotransferase binds pyridoxal-5-P and displays catalytic activity. Enzymatic activity was detected using a sensitive fluorimetric method, which is based on the formation of condensation products between succinic semialdehyde and cyclohexane-1,3-dione. It is concluded that removal of an extra peptide from the precursor is not an obligatory first step in the production of biological active species.

Glutamate decarboxylase side reactions catalyzed by the enzyme

A homogeneous glutamate decarboxylase isolated from pig brain contains 0.8 mol of tightly bound pyridoxal 5-phosphate/enzyme dimer. Upon addition of exogenous pyridoxal 5-phosphate (pyridoxal-5-P), the enzyme acquires maximum catalytic activity. Preincubation of the enzyme with L-glutamate (10 mM) brings about changes in the absorption spectrum of bound pyridoxal-5-P with the concomitant formation of succinic semialdehyde. However, the rate of this slow secondary reaction, i.e. decarboxylative transamination, is 10(-4) times the rate of normal decarboxylation. It is postulated that under physiological conditions enzymatically inactive species of glutamate decarboxylase, generated by the process of decarboxylative transamination, are reconstituted by pyridoxal-5-P produced by the cytosolic enzymes pyridoxal kinase and pyridoxine-5-P oxidase. The catalytic activity of resolved glutamate decarboxylase is recovered by preincubation with phospho-pyridoxyl-ethanolamine phosphate. The experimental evidence is consistent with the interpretation that the resolved enzyme binds the P-pyridoxyl analog, reduces the stability of the covalent bond of the phospho-pyridoxyl moiety, and catalyzes the formation of pyridoxal-5-P.

4-Aminobutyrate aminotransferase. Conformational changes induced by reduction of pyridoxal 5-phosphate

Conformational changes induced in 4-aminobutyrate aminotransferase (4-aminobutyrate:2-oxoglutarate aminotransferase, EC 2.6.1.19) by conversion of pyridoxal-5-P to pyridoxyl-5-P were examined by two independent methods. The reactivity of the SH groups of the reduced enzyme is increased by chemical modification of the cofactor. 1.8 SH per dimer of modified enzyme react with DTNB, whereas 1.2 SH per dimer of the native enzyme react with the attacking reagent under identical experimental conditions. The modified and native forms of the enzyme bind the fluorescent probe ANS, but the number of binding sites for ANS is increased as result of conversion of P-pyridoxal to P-pyridoxyl. After the conformational changes onset by reduction of the cofactor, the modified enzyme binds one molecule of pyridoxal-5-P with a Kd of 0.1 microM to become catalytically competent. The catalytic site of the reduce enzyme was probed with P-pyridoxal analogs. Like resolved 4-aminobutyrate aminotransferase, the reduced species recognize the phosphorothioate analog and regain 40% of the total enzymatic activity. Since the catalytic parameters of reduced and native 4-aminobutyrate aminotransferase are indistinguishable, it is concluded that the additional catalytic site of the reduced enzyme is functionally identical to that of the native enzyme.

Binding of new PLP analogs to the catalytic domain of GABA transaminase

The binding site of Pyridoxal-5-P in 4-aminobutyrate aminotransferase was studied by using analogs of the cofactor. A phosphorothioate analog (PLP(S] recognizes the catalytic site; it forms a stable complex with the apoenzyme (KD = 1nM) and serves as cofactor during catalysis. Replacement of a non-bridged oxygen by sulfur in the phosphate side chain renders a compound which preserves the negative charges needed for correct alignment of the cofactor at the catalytic site. This phosphorothioate analog of PLP can be used to investigate the catalytic site of vitamin B6 dependent enzymes by means of 31P NMR spectroscopy. A bulky P-pyridoxamine derivative, ie, N-4-azido-2-nitrophenyl pyridoxyl-5-P (NANP) competes with natural cofactor for its binding site. Upon illumination, the arylazide of P-pyridoxamine acts as an efficient photolabeling reagent of the protein. A characteristic feature of this photolabeling reagent, ie, its ability to recognize the cofactor binding site, can be exploited to ascertain the chemical nature of amino acid residues at the catalytic site.

4-Aminobutyrate aminotransferase reaction of sulfhydryl residues connected with catalytic activity

4-Aminobutyrate aminotransferase is inactivated by preincubation with N-(1-pyrene)maleimide (mixing molar ratio 10:1) at pH 7. The reaction with N-(1-pyrene)maleimide was monitored by fluorescence spectroscopy and the degree of labeling of the enzyme determined by absorption spectroscopy. The blocking of 2 cysteinyl residues/enzyme dimer is needed for inactivation of the aminotransferase. The time course of the reaction is significantly affected by the substrate alpha-ketoglutarate, which afforded complete protection against the loss of catalytic activity. Trypsin digestion of pyrene-labeled aminotransferase, followed by gel filtration and "fingerprint" analysis, revealed the presence of only one peptide tagged with the fluorescent probe. The reaction of approximately 1.9 SH residues/dimer with iodosobenzoate resulted in enzyme inactivation together with a formation of an oligomeric species of Mr = 100,000 detectable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The cross-linked subunits are dissociated by addition of 2-mercaptoethanol which also restores full catalytic activity. Altogether, these observations are consistent with the concept that inactivation of 4-aminobutyrate aminotransferase by iodosobenzoate proceeds through disulfide bond formation between vicinal cysteinyl residues of the protein. It is postulated that the critical sulfhydryl groups of the enzyme are situated on opposite sides of the dimeric structure at the subunit interfaces.

The ontogeny of a natural food aversion in domestic rats (Rattus norvegicus) and house mice (Mus musculus)

During a 1-hr feeding test, hungry domestic rats (Rattus norvegicus) that had been reared individually from weaning with a domestic house mouse (Mus musculus) were less likely (p less than .05) to feed on the intact carcass of a freshly sacrificed mouse than were rats that had been reared individually with another rat. Likewise, hungry mice that had been reared individually with a rat were less likely (p less than .01) to feed on a dead rat than were mice that had been reared individually with another mouse. Regardless of the social conditions during rearing, hungry mice were more likely to feed on a dead mouse than were hungry rats to feed on a dead rat. Taken together, the present and other findings suggest that the tendency by rats to reject conspecific flesh stems, at least in part, from prior experience with conspecifics and with their own bodies. The experiential factors mediating the tendency by mice to reject conspecific flesh remain unclear.

Historical review of small and microvascular vessel surgery

The history of small and microvessel surgery is reviewed to aid the beginning microvascular surgeon in gaining a perspective of the surgical techniques used on vessels less than 3 mm in diameter. Although microvascular surgery uses sophisticated microscopes, instruments, and sutures, the fundamental operative procedures are similar to those first worked out on larger vessels. The trial and error methods of past surgeons from many different disciplines have led to ever-expanding possibilities for treatment of sick and injured patients and to laboratory investigation of disease processes. We have tried to highlight the major steps in the development of vascular surgery leading up to our present core of microvascular techniques. We recognize that many of these will become obsolete tomorrow; however, in developing the new it is wise to avoid the mistakes of the past.

Visual attention and neuronal firing patterns in the feline pulvinar nucleus of thalamus

In behaving cats, temporal patterns of neuronal firing were studied during slow wave sleep (SWS), motionless quiet wakefulness (QW) coupled with specific direction of the animal's attention, and during bar pressing performance (BP) for milk reward. The analysis was based on relative relations between sequential spike intervals. The strength of the method is based on the fact that the probabilities of occurrence of patterns are determined by the history of a spike train. During SWS, the neuronal firing modes closely followed the theoretical model of independent distribution of intervals, whereas during QW and BP specific for each neuron departures from the model, i.e., patterning was observed. Most importantly, in seven chronically studied neurons idiosyncratic patterns were related to direction of the animal's attention, and, very likely, to the visual forms the animals gazed at, because the patterns disappeared in the dark and during SWS without major changes in the mean firing rate. The replications of patterns upon recurrence of a particular direction of attention was proven statistically. The constancy and idiosyncrasy of these patterns were apparent even though the comparable episodes occurred several hours apart, and the animals slept and/or ate in between, and the distance, i.e., the retinal size of visual forms varied from one episode to another. On the basis of correlative evidence, it was argued that, compared to more abstract modes of information processing, the identification and quantification of patterns based on relative relations between intervals require the least amount of storage of intermediate results. Hence, these patterns are likely to represent a simple and phylogenetically old principle of communication between neurons. It was postulated that the idiosyncrasy and invariance of patterns may play a role in constancy of feature extraction and Gestalt perception.

Death in young alcoholics

Homicide, accidents and heart disease were the leading causes of death among young alcoholics treated at an outpatient alcoholism clinic over a 3-year period. Ways of preventing early death in alcoholics are suggested.