Sept., 1950
COMMUNICATIONS TO THE EDITOR
4333
of cis-aconitate by increase in absorption of light a t 240 mp, using a Beckman spectrophotometer. The assay was made in a quartz absorption cell containing phosphate FROM THE LANKENAU HOSPITAL buffer, PH 7.4, 0.05 M,citrate, 0.03 M , and tissue extract RESEARCH INSTITUTE AND in a total volume of 3 ml. The extract was prepared by THEINSTITUTEFOR homogenizing the tissue with 20 volumes of 0.1 M phosCANCER RESEARCH; AND THE phate buffer, PH 7.4, and centrifuging off the residue. A DEPARTMENT OF CHEMISTRY SIDNEY WEINHOUSE unit of enzyme activity is the amount producing a change TEMPLEUNIVERSITY RUTHH. MILLINCTON in optical density a t 240 mp of 0.001 per minute a t 25”. PHILADELPHIA, PA. CHARLES E. WENNER Isocitric dehydrogenase was determined on extracts of acetone powders by the optical method of Mehler, et a1.,6 RECEIVED JULY27, 1950 a unit of activity being defined as the amount required t o produce a change in optical density of 0.01 per minute a t 25’. Citrate formation in complete system without ENZYMES OF THE CITRIC ACID CYCLE IN tissue was 0.21 pM. Citrate content of all tissues was TUMORS’
