1,721,003 research outputs found
Double Inhibition of L-Threonine Dehydratase by Aminothiols
No full textThe inhibition of highly purified rat liver L-threonine dehydratase (L-threonine hydro-lyase (deaminating), EC 4.2.1.16) by aminothiols (L-cysteine, D-cysteine, cysteamine) has been studied. Single inhibition experiments evaluated by Lineweaver-Burk and Dixon plots showed, in a given concentration range, partially (parabolic) competitive inhibitions, indicating two binding sites for each inhibitor. Double inhibition experiments revealed that the inhibition was antagonistic, the two inhibitors weakening each other's effect. Formation of EI1 and EI2 binary complexes, and ESI1, ESI2 and EI1I2 ternary complexes was demonstrated, while formation of the quaternary complex ESI1I2 was ruled out. It is assumed that one inhibitor-binding site coincides with the substrate-binding center while the second inhibitor-binding (allosteric, regulatory) site may comprise the pyridoxal-phosphate-binding SH group(s). The comparison between Km and Ki values and the evaluation of intracellular concentrations of L-threonine, L-cysteine and cysteamine suggest a possible physiological role of the inhibition
The determination of urinary oxypurines as markers of gastrointestinal tumors
No full textPlasma levels and urinary excretion of oxypurines--hypoxanthine and xanthine--were evaluated by reverse-phase high-pressure liquid chromatography in 13 patients affected by gastric tumors and in 19 colorectal tumor-bearing patients. Preliminary results indicate higher values of urinary xanthine and an increase in the xanthine/hypoxanthine ratio in cancer patients. The increase was not generalized to all subjects, and did not appear related either to the stage of the disease or to CEA values. The limits within which the determination of urinary oxypurines can be employed as a tumor marker are discussed
Effects of starvation, hyperproteic diet and nicotinamide treatment on the L-threonine aldolase and allothreonine aldolase of rat liver [Effetto del digiuno, della dieta iperproteica e del trattamento con nicotinamide sulla treonina e allotreonina aldolasi di fegato di ratto]
No full textIn order to evaluate if the aminoacid L-threonine and its isomer allothreonine are metabolized by a single protein or by two specific enzymatic proteins, in the rat liver, we have studied the behaviour of L-threonine aldolase activity and allothreonine aldolase activity in rats which are injected with nicotinamide (1 g/ Kg) or have received a special diet containing 91% of casein or that have been starved for several days. The L-threonine aldolase enhances its activity in all three cases, while the allothreonine aldolase does not show significative changes of activity. These different behaviours of the two enzymatic activities, support the hypothesis that in the rat liver two proteins are necessary for the metabolism of the L-threonine and its isomer allothreonine
A Kinetic Method for Distinguishing whether an Enzyme has one or two Active Sites for two different Substrates. Rat Liver L-Threonine Dehydratase has a single Active Site for Threonine and Serine
No full textWe have elaborated a kinetic method which allows us to evaluate whether a Michaelis-Menten-type enzyme acting on two different substrates has one or two active sites. This method has been used with the rat liver L-threonine dehydratase, which catalyzes the dehydrative deamination of both serine and threonine. The experimental data can be fitted to the theoretical plot obtained for the case of a single active site
Fatty Acid Composition of Plasma Lipids in Gout
No full textThe changes occurring in the fatty acid composition of the plasma lipids were studied in gout. The major changes consisted in an increase in oleic acid and a decrease in linoleic and arachidonic acids in most lipid fractions of plasma; linoleic acid is unchanged in plasma cholesteryl esters and in FFA while arachidonic acid does not vary only in FFA. These changes are not related either to diet or to the age of patients and are similar to those reported in atherosclerotic and diabetic patients, as well as those with cardiac ischemia. The variations observed are directly influenced by the ratio essential fatty acids/monoenoic acids and all factors affecting this ratio
An improved method for purification of L-threonine deaminase from rat liver
No full textL-threonine deaminase was obtained at a high degree of purity from rat liver. Two main steps were added to the previously reported procedure: cryoprecipitation and chromatofocusing (in the presence of a specific KCl concentration). The purification factor was 3,090 and the specific activity 989. The method is very reproducible and convenient. It gives the highest specific activity and the highest degree of purity of the enzyme recorded to date
Reaction between pyridoxal 5'-phosphate and carbamyl-phosphate [Reazione tra piridossal 5'-fosfato e carbamil fosfato]
No full text
In Vitro Regulation of Rat Liver L-Threonine Deaminase by different effectors
No full textThe regulation of liver L-threonine deaminase by different effectors--bile acids, bile pigments and monocarbon molecules--was investigated. Total inhibition of the enzyme was observed with physiological concentrations of bile acids and biliverdin. Purely competitive inhibition of the holoenzyme by several monocarbon molecules was demonstrated; the mechanism was partially competitive for bicarbonate. Inhibition was more pronounced in the case of the dialyzed enzyme. From the higher Km values for pyridoxal-5'-phosphate (PLP), obtained in the presence of the inhibitors, the results are explained on the basis of interference in the association reaction: apoprotein + PLP----holoenzyme. The various effects determined by bicarbonate may play a specific role in vivo since they occur at physiological concentrations of this compound
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