| The studies described in this dissertation are presented in two sections. The first section describes a study aimed at characterizing the molecular basis for hyperinsulinemia and hyperammonemia in an infant. The hyperinsulinism hyperammonemia syndrome is due to dominant mutations in glutamate dehydrogenase that affect regulation of the enzyme's bioactivity. We identified a novel, gain-of-function missense mutation (H262Y) in the glutamate dehydrogenase gene of a patient with hypoglycemic hyperinsulinemia. Based on structure-function considerations, the mutation is postulated to interfere with GTP suppression of GDH activity.; The second section describes studies aimed at investigating the role of energy expenditure in the response to weight-reduction in mice. We first assessed the utility, accuracy and precision of a non-invasive method (dual-energy X-ray absorptiometry) for measuring body composition of mice. Longitudinal measures of mice before and following body weight reduction demonstrated that the method is suitable for detecting changes in body composition of mice that have lost 15--20% of their initial body weight. We tested the hypothesis that mice maintaining a reduced body weight (15--20% below initial) have lower energy expenditure than predicted after adjusting for body weight and composition. Lean tissue mass adjusted resting energy expenditure tended to be lower in the weight-reduced groups (∼10--14%). However, weight-reduced mice had higher absolute non-resting energy expenditure than controls, despite their smaller body size. Consistent with the observed increase in non-resting energy expenditure, the weight-reduced mice were more physically active than their ad libitum fed controls. When differences in body size were taken into account, weigh-reduced mice fed a low fat diet performed significantly more "work" (estimated by multiplying body weight by distance traveled in chamber) than their ad libitum fed controls. Furthermore, weight-reduced mice demonstrated changes in skeletal muscle gene expression primarily in fast-twitch muscle. The functional significance of these changes would be expected to involve a reduction in electrical excitability and fiber size, in addition to a transition to a slower contractile phenotype. |
