1. The interconversion of hydroxypyruvate and l-glycerate in the presence of NAD and rat-liver l-lactate dehydrogenase has been demonstrated. Michaelis constants for these substrates together with an equilibrium constant have been determined and compared with those for pyruvate and l-lactate. 2. The presence of d-glycerate dehydrogenase in rat liver has been confirmed and the enzyme has been purified 16-20-fold from the supernatant fraction of a homogenate, when it is free of l-lactate dehydrogenase, with a 23-29% recovery. The enzyme catalyses the interconversion of hydroxypyruvate and d-glycerate in the presence of either NAD or NADP with almost equal efficiency. d-Glycerate dehydrogenase also catalyses the reduction of glyoxylate, but is distinct from l-lactate dehydrogenase in that it fails to act on pyruvate, d-lactate or l-lactate. The enzyme is strongly dependent on free thiol groups, as shown by inhibition with p-chloromercuribenzoate, and in the presence of sodium chloride the reduction of hydroxypyruvate is activated. Michaelis constants for these substrates of d-glycerate dehydrogenase and an equilibrium constant for the NAD-catalysed reaction have been calculated. 3. An explanation for the lowered V(max.) with d-glycerate as compared with dl-glycerate for the rabbit-kidney d-alpha-hydroxy acid dehydrogenase has been proposed.
In human serum, l‐tryptophan is the only amino‐acid bound to protein. Salicylate causes a release of tryptophan from its binding sites on human serum proteins and bovine albumin. Some implications of this finding are discussed.
Salicylate inhibits the activities of a number of cellular enzymes and in some instances the mechanisms of inhibition have been established (Smith, 1968a). Reported inhibitory actions of the salicylate ion on important enzyme systems in vitro are now reviewed and assessed in relation to the known clinical and toxic effects of the drug.
Salicylate inhibits rabbit muscle lactate dehydrogenase, horse liver alcohol dehydrogenase, pig heart malate dehydrogenase and pig heart isocitrate dehydrogenase in vitro. The inhibitions are reversible, involving competition with nad, nadh2 or nadp. The results are discussed with reference to some of the in vivo actions of the drug.
SUMMARY Tissue concentrations of 5-HT have been measured in the duodenal mucosa of adults and children with coeliac disease and were found to be significantly higher than those from a control group. This finding may be associated with hyperactivity or hyperplasia of enterochromaffin (EC) cells in the duodenum of patients with coeliac disease and could also be directly related to described abnormalities of 5-HT metabolism in this disease.
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