Effect of orthophosphate on the transaldolase reaction.

In previous studies' with a dialyzed supernatant fraction from rat liver it was observed that the formation of heptulose phosphate from fructose g-phosphate was significantly inhibited by inorganic phosphate. It has since been shown (1, 2) that the activity of the dialyzed liver fraction can be related to its content of transketolase and transaldolase and it became of interest to determine whether the inhibition of the over-all process by phosphate was due to its effect on one or both of the enzymes involved in this process. Preliminary experiments indicated that both transketolase and transaldolase were inhibited by inorganic phosphate when the enzymes were tested in the usual assay procedures. The effect of phosphate on transaldolase is reported in the present paper

Crystalline D -Gluconate 6-Phosphate Dehydrogenase"

A method for t.he purification of n-gluconate 6-phosphate dchydrogenase from brewers' yeast has been described by Horecker and Smyrniotis. (1). The enzyme preparation also catalyzed the oxidation of n-glucose 6-phosphate and isocitrate and in addition contained significant amounts of n-ribose 5-phosphate isomerasc. The utility of the enzyme for the assay of n-gluconate 6-phosphato and for the preparation of n-ribulose 5-phosphate was seriously affected by the prcsencc of thcsc contaminating activitics. For a further investigation of the properties of the enzyme and of the nature of the two-step reaction, a more highly purified preparation was necessary. A method has now been developed which yields from Candida yeast a crystalline preparation which is free of detectable quantitics of I)-glucose 6-phosphata dchydrogenase and r)-ribose 5-phosphate isomerasc. To remove the former enzyme, advantage was taken of its lability to heat after treatment with charcoal (2, 3). n-Glucose 6-phosphate dchydrogcnasc is known to bc unstable in t,he absence of triphosphopyridine nuclcotidc. With the purified enzyme, only n-ribulosc 5-phosphate is formed, and this ester can thus be obtained free of n-ribose 5-phosphate and n-xylulose j-phosphate. Since the crystalline enzyme preparation catalyzes the osidation of reduced triphosphopyridine nucleotide by CO, and o-ribulose phosphate at a rate equal to that observed with crude enzyme preparations, wc have concluded that a single enzyme is rcsponsible for both oxidation and decarboxylation reactions. We have also undertaken an investigation of the specificity of the Mg++ requirement (1) and havn found that at high concentrations of NaCl, full activity is obtained in the absence of Mg++. There is reason to believe that the nature of t,he anion plays a role in the activation of the enzyme

Evidence for the modification of fructose 1,6-bisphosphatase by two distinct lysosomal proteases

none6Pontremoli S; Melloni E; Accorsi A; De Flora A; Salamino F; Horecker BL.Pontremoli, Sandro; Melloni, Edon; Accorsi, A; DE FLORA, Antonio; Salamino, Franca; Horecker, B. L