Prahlad, K. V.
M.S. (Master of Science)
Department of Biological Sciences
Xanthine; Androgens; Enzymes
Xanthine oxidase converts the purine bases xanthine and hypoxanthine to uric acid. This enzyme has been shown to exhibit sex specific differences in which mature male rats have a greater compliment of enzyme activity than mature female rats (Levinson and Chalker, 1981). To investigate the mechanism for development of a greater enzymatic activity in mature male rat liver a sensitive radioimmunoassay was developed to measure enzyme mass. The radioimmunoassay is effective in measuring as little as 5ηg of xanthine oxidase protein and is linear through 250ηg of xanthine oxidase protein. Using this sensitive radioimmunoassay the mature male rat was found to have 20% greater enzyme mass (mean ± SD: male, 2.72 ± .34 μg/mg liver protein; female, 2.26 ± .27 μg/mg liver protein; p <.002) when compared with mature female rats. Furthermore, rat liver xanthine oxidase determined by radioimmunoassay showed a correlation with the specific activity of the enzyme using a radiochemical assay employing an NAD+ regenerating system (r=.843, p<.001). It was also shown that mean values for xanthine oxidase activity were significantly greater in males whether the enzyme was assayed in the oxidase form (0 form) (mean ± SD: male, 227 ± 30 μmole/mg prot. h⁻¹ ; female, 179 ± 18 μmole/mg prot. h⁻¹ ; p<.001) or in the dehydrogenase form (D form) in the presence of NAD+ (mean ± SD: male, 531 ± 70 μmole/mg prot. h⁻¹ ; female, 409 ± 39 μmole/mg prot. h⁻¹ ; p<. 001). The increase in enzyme activity in mature male rat liver xanthine oxidase appears to be due to an increase in the mass of the enzyme. The involvement of sex hormones in these sex specific differences of rat liver xanthine oxidase was also investigated. When male rats were castrated at 2 days of age a 28% decrease in adult xanthine oxidase activity occurred when compared with sham operated controls (p<.001). This decrease in enzyme activity could not be altered or restored by exogenous testosterone in the adult animal (mean ± SE: 2 day castrate 443 ± 5.06 μmole/mg prot. h⁻¹ ; 2 day castrate + testosterone, 465 ± 7.2 μmole/mg h⁻¹ ; p>.05). However, when male rats were castrated at 30 days of age a significant increase in adult xanthine oxidase activity occurred in response to testosterone replacement at 72 days of age (mean ± SE: 30 day castrate, 518.3 ± 9.27 μmole/mg prot. h⁻¹ vs. 30 day castrate + testosterone, 624.6 ± 20.3, p<.002). Male rats exposed to androgen during the neonatal period, therefore, display a heightened degree of androgen responsiveness in the adult as measured by augmented xanthine oxidase activity and mass at sexual maturity. This augmented response in mature male rats is also independent of the pituitary gland. Hypophysectomized males show a 29% decrease in enzyme activity when compared to intact controls (p<.001). However, when hypophysectomized males are given replacement therapy with testosterone a 40% increase in enzyme activity occurred (p<.001). Therefore, an intact pituitary does not seem to be required for full expression of xanthine oxidase activity in male rats which have been exposed to neonatal androgens. The sexual dimorphism of rat live xanthine oxidase appears to occur through an increased enzyme mass of xanthine oxidase in mature male rats, as a result of neonatal adrogen exposure.
Decker, Douglas E., "Regulation of male rat liver xanthine oxidase activity as dependent on the presence of androgen priming in the neonatal period" (1982). Graduate Research Theses & Dissertations. 4688.
Northern Illinois University
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