Isocitrate dehydrogenase activity and its regulation by estradiol in tissues of rats of various ages

The activity and hormonal regulation of NAD‐ and NADP‐linked isocitrate dehydrogenase (EC.1.1.1.41 and EC.1.1.1.42, respectively) in the brain, liver and kidney cortex of female rats of various ages was investigated. The activity of NAD‐ICDH of brain was greater than extramitochondrial (‐c) or intramitochondrial (‐m) NADP‐ICDH. In contrast, liver c‐NADP‐ICDH was much higher than NAD‐ or m‐NADP‐ICDH, whereas in kidney cortex the activity of m‐NADP‐ICDH is dominant over both NAD‐ and c‐NADP‐ICDH in all the age group of rats studied. The activity of the NAD‐ICDH of brain and all the enzymes of liver and kidney cortex increases until adulthood (33‐weeks) and decreases thereafter in old rats (85‐weeks). In brain c‐NADP‐ICDH was much higher in immature (6‐weeks) rats and decreases with increasing age of the animal, whereas m‐NADP‐ICDH showed no significant change with the age of the rats. Bilateral ovariectomy decreases the level of all the three forms of enzyme in all the tissues of 6‐, 13‐ and 33‐week rats but failed to show any significant effect in 85‐week old rats. Exogenous administration of estradiol induces all the three forms of enzyme in all the tissues of ovariectomized rats. The degree of response is tissue‐ and age‐specific.