. Cell chemistry; a collection of papers dedicated to Otto Warburg on the occasion of his 70th birthday. Warburg, Otto Heinrich, 1883-; Biochemistry. 228 I. LIEBERMAN, A. KORNBERG VOL. 12 (1953) Specificity of DPNH and influence of DPNH concentration on orotate reduction. With the purified enzyme preparation no observable reduction of orotate occurred in the absence of added DPN. With TPN, M X 10^ orotate CS) , . , , . , 5 10 15 20 25 30 the Tcaction rate was Icss than 2 % of that observed with DPN, and no inhibiting effect on the reaction (with DPN) was noted. The rate of orotate reduct
. Cell chemistry; a collection of papers dedicated to Otto Warburg on the occasion of his 70th birthday. Warburg, Otto Heinrich, 1883-; Biochemistry. 228 I. LIEBERMAN, A. KORNBERG VOL. 12 (1953) Specificity of DPNH and influence of DPNH concentration on orotate reduction. With the purified enzyme preparation no observable reduction of orotate occurred in the absence of added DPN. With TPN, M X 10^ orotate CS) , . , , . , 5 10 15 20 25 30 the Tcaction rate was Icss than 2 % of that observed with DPN, and no inhibiting effect on the reaction (with DPN) was noted. The rate of orotate reduction ap- peared to be dependent on DPNH concen- tration since it was found to be propor- tional to the amount of glucose dehydro- genase when this enzyme was added in limiting quantities. In one experiment, with units of purified enzyme, the decrease in optical density at 280 mju, in 4 minutes was found to be , , , and , with , 125, 250, and 310 units of glucose dehydrogenase, respectively. With 4 units or less of di- hydro-orotic dehydrogenase, glucose de- hydrogenase at a level of 250 units did not limit the rate of the reaction. The concentration of glucose used in the assay system was ^^' ^^^ it appeared to be sufficient to allow a maximum rate of orotate reduction. Decreasing the con- centration of glucose to M caused a decrease of less than 10% in the reaction rate. Approximately half the maximum rate was obtained with M glucose. Specificity of dihydro-orotic dehydro- genase and the influence of orotate concen- tration on rate of reaction. The possibility that dihydro-orotic dehydrogenase can catalyze the reduction of other pyrimidines was investigated. No activity and no inhibition of orotate reduction was observed with uracil, cytosine, 5-methylcytosine, or thymine. The rate of orotate reduction was studied as a function of orotate concentration (Fig. i). When the data were plotted according to Lineweaver and Burk-", as
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