Changes in the serum LDH isoenzymes in monkey during chronic exposure to simulated high altitude

When the monkey was exposed to a simulated high altitude of 18,000 ft (5,486 m) for 30 d, the serum LDH activity was increased to its maximum in 1 week. After the monkey returned to sea level, the enzyme activity showed a rapid recovery. On the basis of electrophoretic analysis of LDH isoenzymes, the percentages of LDH-1 and LDH-2 were decreased during the first 2-3 weeks of hypoxic exposure whereas those of LDH-3, LDH-4 and LDH-5 were increased. After 2 or 3 weeks of hypoxic exposure, LDH-3, LDH-4, and LDH-5 become predominant, indicating that the isoenzyme pattern shifted to an anaerobic form from an aerobic form. These results clearly showed that relative proportions of the five isoenzymes were significantly altered by chronic hypoxia of high altitude and that the elevation of total serum LDH activity induced by hypoxia came predominantly from rapid accumulation of M type LDH isoenzymes in serum.