A B S T R A C T Monoclonal antibodies were prepared against pyruvate kinase (PyKi; ATP: pyruvate phosphotransferase, EC 2.7.1.40) and used to quantitate PyKi content in L2 lung cells and WI-38 fibroblasts cultivated under hypoxic and normoxic conditions. After 96 h of hypoxic cultivation, PyKi activity was significantly increased in both cell types (L2: normoxia [Po2 = 142 torr], 0.11+0.01 [SD]; hypoxia [Po2 = 14 torr], 0.25 ±0.04 U/,ug DNA, P < 0.01). PyKi content increased proportionately in both cell lines (L2: normoxia, 0.44 +0. 13; hypoxia, 0.94±0.13 ,g enzyme protein/,Lg DNA).
Specific activity was not significantly different after 96h (L2: normoxia, 261±11; hypoxia, 261+14 U/mg enzyme protein). These results indicate that regulation of glycolysis during chronic hypoxia occurs at the level of enzyme content. Chronic 02 depletion leads to either an increased rate of biosynthesis or a decreased rate of biodegradation of PyKi, causing augmented glycolytic capacity. Monoclonal antibodies provide a highly specific, convenient approach to characterizing enzymes, as well as quantitating cellular enzyme content.
INTRODUCTIONChronic 02 depletion is an important clinical problem, but the cellular mechanisms responsible for adaptation to chronic hypoxia are obscure. The increased rate of glycolysis following acute hypoxic exposure was first described by Pasteur [3][4][5]. This stimulation ofglycolysis by chronic hypoxic exposure may also explain the bioenergetic differences observed between pulmonary artery endothelial cells (Po2 = 40 torr) and aortic endothelial cells (Po2 = 100 torr), as well as the differences observed in fetal and neonatal brain (6,7). Increased activity of rate-limiting glycolytic enzymes has also been found in fibroblast, endothelial, and kidney cell lines cultured under hypoxic conditions. Therefore, the phenomenon appears to be a common cellular response to 02 depletion. To investigate the mechanism by which the activity of PyKi is increased following chronic hypoxic exposure, we determined the isozyme type and content of PyKi present in a cloned cell line derived from rat lung (L2) and in WI-38 fibroblasts following hypoxic or nonnoxic cultivation. The results show an increase in the intracellular content of PyKi, with no significant change in specific activity (PyKi activity/PyKi content) or isozyme type present after hypoxic cultivation. This suggests that the availability of molecular 02 modulates the content of key glycolytic enzymes, which in turn regulate the generation of ATP through glycolysis.
METHODSCell culture. L2 clone derived from rat lung was obtained from Dr. William Douglas in 36th population doubling (8) and maintained in cell culture by using Ham's F-12 medium containing 10% fetal calf serum (FCS), 100 U/ml penicillin, and 100 ,ug/ml streptomycin. Confluent plates were trypsinized and replated weekly with a 1:4 split, and experiments were performed on cells between the 40th and 55th population doubling.WI-38 fibroblasts were obtained from the American Type Culture Collect...