BEHAVIOR OF TRANSALDOLASE (EC-2.2.1.2) AND TRANSKETOLASE (EC-2.2.1.1) ACTIVITIES IN NORMAL, NEOPLASTIC, DIFFERENTIATING, AND REGENERATING LIVER

Google Scholar
  • 1 January 1976
    • journal article
    • research article
    • Vol. 36  (9) , 3189-3197
Abstract
The objective of this investigation was to throw light on the biological behavior and metabolic regulation of hepatic enzymes of the nonoxidative branch of the pentose phosphate pathway. The activities of transaldolase (EC 2.2.1.2) and transketolase (EC 2.2.1.1) were compared in biological conditions that involve modulation of gene expression such as starvation, differentiation, after partial hepatectomy and in a spectrum of hepatomas of different growth rates. The enzyme activities were determined under optimal kinetic conditions by spectrophotometric methods in the 100,000 .times. g supernatant fluids prepared from tissue homogenates. The kinetic properties of transaldolase and transketolase were similar in normal liver and rapidly growing hepatoma 3924A. For transaldolase, apparent Km values of 0.13 mM (normal liver) and 0.17 mM (hepatoma) were observed for erythrose-4-phosphate and of 0.30-0.35 mM for fructose-6-phosphate. The pH optima in liver and hepatoma were at approximately 6.9-7.2. For the transketolase substrates, ribose-5-phosphate and xylulose-5-phosphate, the apparent Km values were 0.3 and 0.5 mM, respectively, in liver and hepatoma. A broad pH optimum around 7.6 was observed in both tissues. In organ distribution studies, enzyme activities were measured in liver, intestinal mucosa, thymus, kidney, spleen, brain, adipose tissue, lung, heart and skeletal muscle. Taking the specific activity of liver as 100%, transaldolase activity was the highest in intestinal mucosa (316%) and in thymus (219%). It was the lowest in heart (53%) and skeletal muscle (21%). Transketolase activity was highest in kidney (155%) and lowest in heart (26%) and skeletal muscle (23%). Starvation decreased transaldolase and transketolase activities in 6 days to 69 and 74%, respectively, of those of the liver of the normal, fed rat. This was in the same range as the decrease in protein concentration (66%). In liver tumors, transaldolase activity was increased 1.5-3.4 fold over the activities observed in normal rat liver. Transketolase activity showed no relationship to tumor proliferation rate. In the regenerating liver at 24 h after partial hepatectomy, the activity of both pentose phosphate pathway enzymes was in the same range as that of sham-operated controls. In differentiation at the postnatal age of 5, 12, 23 and 32 days, hepatic transaldolase activities were 33, 44, 55 and 72%, respectively, of the activities observed in the 60 day old, adult male rat. During the same period, transketolase activities were 18, 21, 26 and 55% of the activities observed in liver of adult rat. The demonstration of increased transaldolase activity in hepatomas, irrespective of the degree of tumor malignancy, differentiation or growth rate, suggests that the reprogramming of gene expression in malignant transformation is linked with an increase in the expression of this pentose phosphate pathway enzyme. Since no similar alteration was found in differentiating or regenerating liver, the increased transaldolase activity appears to be specific to neoplastic transformation. The increase in transaldolase activity in conjunction with the earlier observed increase in glucose-6-phosphate dehydrogenase activity should provide selective advantages to neoplastic cells.