Deamination Of Leucine Research Articles

COMPARISON OF THE ENZYME OXIDIZING THYROID HORMONE WITH L-AMINO ACID OXIDASE.

An oxidase, which catalyzes mainly the deamination of thyroxine, 3,3′,5-triiodothyronine and 3,5-diiodothyronine, has been partially purified from rat-kidney mitochondrial extracts by successive (NH4)2SO4 fractionation and diethylaminoethyl-cellulose-column treatment. The enzyme preparation was contaminated with catalase (H2O2: H2O2 oxidoreductase, EC 1.11.1.6) in relatively high concentration, but contained no thyroid-hormone transaminase. The enzyme preparation had no effect on either diiodotyrosine or tyrosine, leucine and lactic acid were well oxidized with the formation of keto acids. The elution pattern of the partially purified oxidase on diethylaminoethyl-cellulose column indicates that the enzyme oxidizing thyroid hormone is identical to the mammalian l-amino acid oxidase (l-amino acid: O2 oxidoreductase, EC 1.4.3.2) which catalyzes chiefly the deamination of leucine and the dehydrogenation of lactic acid. Furthermore, the prosthetic group of the purified enzyme oxidizing thyroid hormone was identified as riboflavin 5′-phosphate, which is well known as the prosthetic group of the mammalian l-amino acid oxidase.

Presence and metabolism of amino acids in young and old lenses

By means of automatically recording ion exchange chromatography, 30 non-protein-bound, ninhydrin-positive compounds in a 24-hr acid-hydrolysate of perchloric acid extracts of rat, calf and bovine lens were determined. A microapparatus is mentioned, which permits complete analyses with 100–200 mg tissue. Most amino acids are less concentrated in old lenses than in young ones. Methionine, glucosamine and hydroxyproline form an exception. After incubation of young and old rabbit lenses with [ 35 S]—methionine, seven 35 S—containing metabolites could be identified. The lens is capable of degrading methionine to taurine and SO 4 — . The activity of the glutamic-oxaloacetic transaminase is comparatively high, whereas low activity of glutamic-pyruvic transaminase is found only in the epithelium. Experiments on the deamination of various amino acids revealed practically no deamination of leucine and alanine, while glutamic acid and arginine were rapidly deaminated. After incubation with arginine of young and old lenses, ammonia, urea and ornithine could be detected. The production of 17 molecules of urea by 1 molecule of ornithine, and the fact that ornithine does not inhibit the formation of urea from arginine, led to the assumption of the presence of a Krebs-Henseleit urea cycle in the lens.