Comparison of phosphohexose isomerase and lactic dehydrogenase activities in plasma, liver, and tumor tissue of tumor-bearing rats.

The mechanisms underlying increases in the activity of various enzymes in the serum or plasma are, in general, incompletely understood. It has been stated that the plasma enzymes are derived from cellular elements either in the tissue or in the blood and that the level of circulating enzymes is therefore an index of the rate of disintegration of such cells (8). On the other hand, in some instances an elevation in activity of a particular plasma or serum enzyme is specifically related to a tissue process involving that enzyme, as, for example, the high serum alkaline phosphatase associated with increased osteoblastic activity in bone or with ob struction of the biliary system. In 1943, Warburg and Christian (19) postulated that the excessive glycolytic activity of tumors might be manifested by increased activity of vari ous glycolytic enzymes in the serum. In pursuance of this concept, several investigations have ex plored the serum levels of aldolase (1, 15-17, 19), phosphohexose isomerase (4—6), and lactic dehydrogenase (2, 10) in animals with transplanted tu mors or in patients with neoplastic disease. Several concepts concerning the factors controlling the level of serum glycolytic enzyme activity have been suggested as a result of those studies (5,6,16) It was believed that further information con cerning these mechanisms might be obtained by comparing the changes occurring in the activities of two glycolytic enzymes in the serum with the changes in the neoplastic tissue, liver, and erythrocytes during the growth of a transplanted