Limiting factors for glycogen storage in tumors. I. Limiting enzymes.

The paper presents a study of various enzymes involved in the final accumulation of glycogen in tumor tissues. The solid tumors used are : Novikoff hepatoma, Walker 256 carcinosarcoma, Sarcoma 37, leukemia, and melanoma. It was observed that tumor tissues showed lower activity of phosphoglucomutase and glycogen synthetase as compared with that of normal liver and muscle. All tumors studied except melanoma also showed decreased UDPG' pyrophosphorylase levels. Activities for ATPand UTP-regenerating enzymes—namely, pyruvate kinase and nucleoside disphospho kinase—in tumors were close to those found in normal tissues, thus providing sufficient quantities of cofactors (ATP and UTP) and energy for the polymerization process. UDPG was not diverted to UDPGA in tumors because of the absence of UDPG dehy drogenase. The lesion could be best described as a defective system for glycogen syn thesis, owing to low activities of the enzymes involved in the synthetic process (phos phoglucomutase and glycogen synthetase) which were unable to accomplish efficient transformation of G-6-P to G-1-P and of UDPG to glycogen, in the presence of com peting high rates of tumor glycolysis and normal polysaccharide degradation by phosphorylase.