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Biomedical subjects

K F Tipton

Publications and source records attributed to K F Tipton.

At least 127 records · Page 7Linked to original sources
PubMed1990

Monoamine oxidase and semicarbazide-sensitive amine oxidase activities in bovine eye.

The activities of MAO-A, MAO-B and the semicarbazide-sensitive amine oxidase (SSAO) were determined in bovine lens, retina, optic nerve, iris and epithelium. No activities were detected in lens but the SSAO activity was found to be rather evenly distributed in the other tissues. The SSAO activities determined with 1 microM benzylamine were about half those determined for MAO-B. MAO-A activity, measured with 5-HT as substrate, was considerably greater than those of MAO-B and SSAO, determined with 2-phenylethylamine and benzylamine, respectively, in all the eye tissues.

Amine Oxidase (Copper-Containing)↗
PubMed1990

Amine oxidase activities in bovine lung.

The subcellular distributions of amine oxidase activities MAO-A, MAO-B and SSAO, in bovine lung have been determined. MAO activities are largely confined to the mitochondria whereas SSAO activity is concentrated in the microsomal fraction. The contribution of different type of amine oxidase present in the microsomal fraction to the metabolism of the benzylamine, tyramine, phenylethylamine, tryptamine and dopamine were determined. The kinetic constants towards benzylamine showed SSAO to have a higher affinity than MAO-B, and the comparison of the Vmax/Km relationship for both enzymes indicated that benzylamine would be oxidised more efficiently by SSAO than by MAO-B.

Amine Oxidase (Copper-Containing)↗
PubMed1990

Behaviour and properties of catechol-O-methyltransferase from human placenta.

A procedure is reported for the purification of human placental catechol-O-methyltransferase. The preparation is apparently homogeneous and behaves as a monomer with an approximate Mr of 23,000. The sequence of the first 21 amino acid residues from the N-terminal end of the protein is reported. The activity of the enzyme is strongly influenced by the nature of the buffer in which it is assayed.

Amino Acid Sequence↗
PubMed1989

Deuterium isotope effect of phenelzine on the inhibition of rat liver mitochondrial monoamine oxidase activity.

Phenelzine is a suicide monoamine oxidase (MAO) inhibitor with antidepressant properties. The present study compares the inhibition of rat liver mitochondrial MAO by phenelzine and 1,1-dideuterated phenelzine and the metabolism of these drugs by that enzyme. Phenylacetaldehyde, which was measured by a high performance liquid chromatographic procedure, was found to be the major metabolite of phenelzine after incubation with MAO. The time-courses of aldehyde formation were non-linear due to the time-dependent inhibition of MAO. The reaction rate was reduced substantially when the hydrogen atom in the 1-carbon position was replaced by deuterium. The VH/VD value was 3.1, indicating a primary isotope effect. Such a substitution of deuterium in the phenelzine molecule did not affect significantly the initial reversible inhibition of MAO, which was determined by comparison of their Ki values. The irreversible inhibition, as estimated from IC50 values, however, was potentiated substantially by deuteration. These results support the notion that the irreversible inhibition of MAO activity by phenelzine proceeds via a phenylethyldiazene intermediate, which reacts with the enzyme to form a covalent adduct. An alternative pathway involving hydrogen abstraction from carbon-1 of phenelzine or via rearrangement of the diazine on the enzyme surface could occur to form a phenylethylidene hydrazine intermediate which would subsequently be hydrolyzed to phenylacetaldehyde. The reduction in the rate of phenylethylidene hydrazine formation due to the isotope effect could lead to the accumulation of phenylethyldiazene intermediate and thus potentiate the inhibition of MAO activity.

Acetaldehyde↗
PubMed1989

The effects of phospholipids on the activation of glutamate dehydrogenase by L-leucine.

Glutamate dehydrogenase in disrupted mitochondrial preparations is activated by L-leucine to a much greater extent than is the purified enzyme. A factor, or factors, responsible for modulating the sensitivity of L-leucine is lost during the purification of the enzyme. Although both cardiolipin and phosphatidylserine are inhibitors of the enzyme, only the inhibition by the former phospholipid is reversed by L-leucine. The inhibition of glutamate dehydrogenase by its binding to cardiolipin in the disrupted mitochondrial preparations and its relief by L-leucine could account for the greater sensitivity of such preparations to activation by that amino acid.

Animals↗
PubMed1989

Activation of glutamate dehydrogenase by L-leucine.

The activation of glutamate dehydrogenase (L-glutamate: NAD(P)+ oxidoreductase (deaminating), EC 1.4.1.3) by L-leucine has been studied. Apparently homogeneous preparations from ox liver and brain were found to respond similarly. Commercially obtained preparations of the enzyme, which had suffered limited proteolysis during the purification procedure, were shown to behave similarly to preparations which had not suffered such proteolysis when the effects of L-leucine on the oxidative deamination reaction were studied using either NAD+ or NADP+ as the coenzyme. There was also no significant difference in the responses when the reductive reaction was determined with NADPH or with 40 microM NADH. At higher concentrations of NADH (160 microM) the unproteolysed preparations were activated by L-leucine to a considerably greater extent than those which had suffered limited proteolysis. These results accord with the greater sensitivity of the former preparations to inhibition by high concentrations of NADH and the relief of such inhibition by L-leucine. This amino acid was also found to relieve the inhibition of the enzyme by GTP, resulting in an apparent increase in the activation observed in the presence of this nucleotide. In contrast, under the conditions used in this work, the apparent degree of activation by L-leucine was found to be decreased in the presence of the activators ATP or ADP. The presence of high concentrations of NADH (200 microM) potentiated the high substrate inhibition by 2-oxoglutarate, and L-leucine significantly reduced this effect. The effects of L-leucine on the activity of glutamate dehydrogenase thus appear to be composed of a direct effect on the activity of the enzyme together with a relief of high substrate inhibition. The effects of GTP and 2-oxoglutarate in potentiating inhibition by NADH can account for their effects in enhancing the apparent activation by L-leucine. The marked differences in the responses of proteolysed and unproteolysed preparations of the enzyme result from the effects of proteolysis in decreasing the sensitivity to high concentrations of NADH.

Adenosine Triphosphate↗
PubMed1989

Determination of the absolute concentrations of monoamine oxidase A and B in human tissues.

The concentrations of monoamine oxidase-A and -B were determined in homogenates of human cerebral cortex, caudatus and placenta and in human platelet-rich plasma and platelet membranes by determining the specific binding of tritium-labelled pargyline. The concentrations of the two enzyme forms were similar in both human brain regions examined. Determinations of the minimum quantities of clorgyline, (-)-deprenyl, J-508 or pargyline necessary to give complete inhibition of enzyme activity was found to give overestimates of the amounts of monoamine oxidase-A and -B present due to nonspecific binding of these inhibitors.

Autopsy↗
PubMed1988

Purification and characterization of rat intestinal peroxidase. Its activity towards 2-t-butyl-4-methoxyphenol (BHA).

Impure preparations of rat intestinal peroxidase were shown to aggregate at low ionic strengths and to disaggregate at higher values. This aggregation was accompanied by a decrease in specific activity, which could lead to hysteretic behaviour of reaction progress curves. Advantage was taken of this reversible aggregation to obtain a relatively pure extract, which was subsequently purified to apparent homogeneity by affinity chromatography on concanavalin A-Sepharose followed by hydrophobic chromatography. The purified enzyme did not show the ionic-strength-dependent aggregation behaviour, behaving as a monomer of Mr 50,000. The purified enzyme was shown to catalyse the peroxidatic conversion of the commonly used antioxidant 2-t-butyl-4-methoxyphenol (butylated hydroxyanisole, BHA) to form 3,3'-di-t-butyl-2,2'-dihydroxy-5,5'-dimethoxybiphenyl, with a Km value of 176 microM and a maximum velocity of 8 mumol/min per mg. The specificity constant, kcat./Km, for this substrate was similar to that shown towards the substrate guaiacol.

Animals↗
PubMed1988

The involvement of intestinal monoamine oxidase in the transport and metabolism of tyramine.

The monoamine oxidase activities were found to be similar in the crypt and villous cells of the different regions of rat small intestine. In all cases the activity of the A-form of the enzyme constituted more than 70% of the total. A similar proportion of that form of the enzyme was found in homogenates of biopsy samples of human intestine. Studies with everted intestines showed that at concentrations above 10 microM over 70% of tyramine was deaminated during transport and the use of selective inhibitors confirmed the A-form of monoamine oxidase to play the dominant role in that process.

Animals↗