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S B Martin

Publications and source records attributed to S B Martin.

28 records · Page 2Linked to original sources
PubMed1986

Activation of glutamate apodecarboxylase by succinic semialdehyde and pyridoxamine 5'-phosphate.

Glutamate apodecarboxylase was activated by incubation with succinic semialdehyde and pyridoxamine 5'-phosphate. Activation required both compounds and was highly selective for succinic semialdehyde. Of 18 analogs tested, only glyoxylate, pyruvate, oxaloacetate, and 2-oxoglutarate activated the apoenzyme significantly, but much higher concentrations of these compounds than of succinic semialdehyde were required. In the presence of pyridoxamine 5'-phosphate, the concentration of succinic semialdehyde giving half-maximal activation of apoenzyme was 7 microM. In contrast, the Ki for succinic semialdehyde as a competitive inhibitor of glutamate decarboxylation was 1.2 mM, indicating that apoenzyme with bound pyridoxamine 5'-phosphate has a much higher affinity for succinic semialdehyde than does holoenzyme. The concentration of pyridoxamine 5'-phosphate giving half-maximal activation was 17 microM, which is more than an order of magnitude greater than the corresponding value for pyridoxal 5'-phosphate.

Apoenzymes↗
PubMed1985

Transaminations catalysed by brain glutamate decarboxylase.

In addition to normal decarboxylation of glutamate to 4-aminobutyrate, glutamate decarboxylase from pig brain was shown to catalyse decarboxylation-dependent transamination of L-glutamate and direct transamination of 4-aminobutyrate with pyridoxal 5'-phosphate to yield succinic semialdehyde and pyridoxamine 5'-phosphate in a 1:1 stoichiometric ratio. Both reactions result in conversion of holoenzyme into apoenzyme. With glutamate as substrate the rates of transamination differed markedly among the three forms of the enzyme (0.008, 0.012 and 0.029% of the rate of 4-aminobutyrate production by the alpha-, beta- and gamma-forms at pH 7.2) and accounted for the differences among the forms in rates of inactivation by glutamate and 4-aminobutyrate. Rates of transamination were maximal at about pH 8 and varied in parallel with the rate constants for inactivation from pH 6.5 to 8.0. Rates of transamination of glutamate and 4-aminobutyrate were similar, suggesting that the decarboxylation step is not entirely rate-limiting in the normal mechanism. The transamination was reversible, and apoenzyme could be reconstituted to holoenzyme by reverse transamination with succinic semialdehyde and pyridoxamine 5'-phosphate. As a major route of apoenzyme formation, the transamination reaction appears to be physiologically significant and could account for the high proportion of apoenzyme in brain.

Aminobenzoates↗
PubMed1975

Effect of clonidine on growth hormone, prolactin, luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone in the serum of normal men.

Clonidine (0.15 mg iv), a selective noradrenergic receptor agonist, increased serum growth hormone (GH) levels (greater than 6 ng/ml) on 8 out of 12 administrations to 6 normal men. This increase was independent of the hypotensive effects of the drug and unrelated to changes in serum cortisol. Clonidine induced a hyperglycemic effect in all subjects which was greatest 15 min after commencint the injection. No changes in blood sugar or GH occurred after placebo injection. Apomorphine, a selective dopamine receptor agonist, elevated GH in each of these 6 subjects (greater than 10 ng/ml). Clonidine had no effect on serum prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), or thyroid-stimulating hormone (TSH). These data are compatible with a dual dopaminergic and noradrenergic mechanism modulating GH secretion in normal men and with the absence of a noradrenergic mechanism in the regulation of PRL, LH, FSH, or TSH.

Adult↗
PubMedDate not supplied

Structural features and regulatory properties of the brain glutamate decarboxylases.

It is widely recognized that the two major forms of GAD present in adult vertebrate brains are each composed of two major sequence domains that differ in size and degree of similarity. The amino-terminal domain is smaller and shows little sequence identity between the two forms. This domain is thought to mediate the subcellular targeting of the two GADs. Substantial parts of the amino-terminal domain appear to be exposed and flexible, as shown by proteolysis experiments and the locations of posttranslational modifications. The carboxyl-terminal sequence domain contains the catalytic site and shows substantial sequence similarity between the forms. The interaction of GAD with its cofactor, pyridoxal-5' phosphate (pyridoxal-P), plays a key role in the regulation of GAD activity. Although GAD(65) and GAD(67) interact differently with pyridoxal-P, their cofactor-binding sites contain the same set of nine putative cofactor-binding residues and have the same basic structural fold. Thus the cofactor-binding differences cannot be attributed to fundamental structural differences between the GADs but must result from subtle modifications of the basic cofactor-binding fold. The presence of another conserved motif suggests that the carboxyl-terminal domain is composed of two functional domains: the cofactor-binding domain and a small domain that closes when the substrate binds. Finally, GAD is a dimeric enzyme and conserved features of GADs superfamily of pyridoxal-P proteins indicate the dimer-forming interactions are mediated mainly by the carboxyl-terminal domain.

Animals↗
PubMedDate not supplied

Molecular identification of the turf grass rapid blight pathogen.

Rapid blight is a newly described disease on turf grasses, primarily found on golf courses using suboptimal water for irrigation purposes. On the basis of shared morphological characteristics, it has been proposed that the rapid blight pathogen belongs to a genus of stramenopiles, Labyrinthula, which had been known to cause disease of marine plants only. We have collected 10 isolates from four species of turf grass in five states and sequenced portions of the SSU (18S) rDNA gene from each to provide a definitive taxonomic placement for rapid blight pathogens. We also included sequences from Labyrinthuloides yorkensis, Schizochytrium aggregatum, Aplanochytrium sp., Thraustochytrium striatum, Achlya bisexualis and several nonturf-grass isolates of Labyrinthula. We found that rapid blight isolates indeed are placed firmly within the genus Labyrinthula and that they lack detectable genetic diversity in the 18S rDNA region. We propose that the rapid blight pathogens share a recent common ancestor and might have originated from a single, infected population.

Genes, rRNA↗