Mammalian Liver Enzyme Research Articles

The oxidative dealkylation of insecticidal phosphoric acid triesters by mammalian liver enzymes

1. The dealkylation of the insecticidal phosphoric acid triester, 2-chloro-1-(2,4-dichlorophenyl)vinyl diethyl phosphate, proceeds in mammalian liver slices via an oxidative mechanism and not by hydrolysis. 2. The enzyme that catalyses the reaction is located in the microsomal fraction of liver homogenate and is dependent for activity on molecular oxygen and NADPH. 3. There are large species differences between rat, mouse, rabbit and dog in the activity of the enzymes, the relative rates of dealkylation being 1, 8, 24 and 88 respectively in liver slices. 4. Dimethyl and di-isopropyl phosphate triesters are also dealkylated by rabbit liver microsomal preparations. 5. The mechanism of dealkylation involves hydroxylation at the alpha-carbon atom of an alkyl group, which is removed as the corresponding aldehyde, and is thus analogous to that of similar reactions catalysed by the microsomal mixed-function oxidases. 6. The relevance of these findings in the toxicology of phosphoric acid triesters is discussed.

On the genetic etiology of scurvy.

SummaryScurvy, now regarded as a nutritional disorder due to the lack of the trace food constituent, vitamin C, is shown to be the end result of a typical genetic disease. This genetic disease syndrome has been namedHypoascorbemia. Its primary cause is the hereditary lack of — or defect in — the gene controlling the synthesis of the enzyme, L-gulonolactone oxidase. This is a mammalian liver enzyme, the last one in the series for converting glucose into ascorbic acid. Man is one of the few mammals that lacks this enzyme and hence is unable to sythesize his own ascorbic acid. The gene defect occurred during the course of evolution by a conditional lethal mutation. The replacement of the present vitamin C theory regarding the etiology of scurvy by this genetic concept gives important new viewpoints to the quantitative aspects of ascorbic acid in human physiology and also provides new rationales for the use of high levels of ascorbic acid in normal physiology and in the therapy of clinical entities other than scurvy.

Purification and properties of homogentisate oxygenase from Pseudomonas fluorescens.

Summary Homogentisate oxygenase from Pseudomonas ftuorescens adapted to tyrosine has been isolated, purified and crystallized. The crystallized enzyme was homogeneous on ultracentrifugation, and its sedimentation constant was found to be 11.8 S. The molecular weight of the enzyme was calculated to be about 380 000. Like the mammalian liver enzyme, bacterial homogentisate oxygenase requires ferrous iron as a cofactor. For maximal activity, the enzyme requires at least 10 min preincubation with ferrous iron at pH 6.0. The optimal pH is 6.0. Both glutathione and ascorbate are also required for maximal activity at pH 6.0, but only the former is essential at pH 5.4. The Km values are 6 · 10−4 M for homogentisate and 1 · 10−4 M for Fe2+ at pH 6.0. Unlike the mammalian liver enzyme, the bacterial enzyme is fairly stable on aging or storage. p-Chloromercuribenzoate inhibits the enzyme with respect to ferrous iron, but non-competitively with respect to homogentisate. The properties of bacterial homogentisate oxygenase are discussed in comparison with those of the mammalian liver enzyme.

Effect of ethanol on ethylene glycol oxidation by mammalian liver enzymes.

Effect of ethanol on ethylene glycol oxidation by mammalian liver enzymes.

Differential response of inducible mammalian liver enzymes to in vivo stimulation by ribonucleic acid and a nucleotide mixture

Stimulation of tyrosine-α-ketoglutarate transaminase activity 3- to 5-fold above the basal level is accomplished by intraperitoneal injection into intact rats of ribonucleic acid, nucleotide mixture, and hydrocortisone alone and in various combinations. These substances increase tryptophan pyrrolase activity only about 2-fold over the basal level. In adrenalectomized animals, ribonucleic acid or nucleotide mixture alone are ineffective in raising the level of activity above the basal level for either enzyme; hydrocortisone raises the levels 2- to 4-fold. The combinations of hydrocortisone plus ribonucleic acid or hydrocortisone plus nucleotide mixture raise tyrosine-α-ketoglutarate transaminase activity above that produced by hydrocortisone alone, but the level of tryptophan pyrrolase activity is not raised by these combinations above that produced by hydrocortisone alone.

The action of mammalian liver enzyme preparations on asparagine and asparagine derivatives

α-N-alkyl derivatives of DL-asparagine were not hydrolyzed by rat-liver asparaginase or guinea pig-liver asparaginase. They markedly inhibited the action of these enzymes on L-asparagine. In all cases tested the inhibition was found to be competitive. Rat and guinea pig liver asparaginase did not hydrolyze N-(β- DL-aspartyl)-alkylamines and were inhibited by these compounds only slightly, if at all. The asparagine-α-keto acid transamination-deamidation system did not act on both kinds of asparagine derivatives and was not inhibited by them. Rat-liver preparations which exhibited asparaginase activity also catalyzed β-aspartyl transfer from asparagine to hydroxylamine. The specificity of the transfer reaction towards both kinds of asparagine derivatives was the same as in the case of asparaginase.

Transglucosylation by a Mammalian Liver Enzyme

Transglucosylation by a Mammalian Liver Enzyme