Regulation of expression of genes for enzymes of the mammalian urea cycle in permanent cell-culture lines of hepatic and non-hepatic origin

Effects of dietary vitamin E and selenium on arginase activity in the liver, kidneys, and heart of rats treated with high doses of glucocorticoid

The effects of dietary intake of vitamin E and selenium on arginase activity in the liver, kidneys, and heart of rats treated with high doses of prednisolone were investigated. Rats were divided into five groups. Groups 3, 4, and 5 received a daily supplement in their drinking water of vitamin E, Se, and a combination of vitamin E and Se, respectively, for 30 days. For 3 days subsequently, the control group (group 1) was given a placebo, and the remaining four groups were injected intramuscularly with prednisolone. The tissue samples were collected from each group at 4, 8, 12, 24, and 48 h after the last administration of prednisolone. In the group treated with prednisolone alone, arginase activity in the liver was found to have increased at all the time periods, whereas it had decreased significantly in the heart at 48 h. Arginase activity in the kidneys was not affected by prednisolone. Compared to the control and prednisolone groups, arginase activity in the kidneys and heart of the vitamin E- and Se-supplemented groups was found to be significantly increased at all time periods, however, no difference was seen in the combination group. Arginase activity in the liver of the vitamin E-supplemented group was found to have decreased at all time periods, however, in the Se group compared to the prednisolone group it had reduced at 24 and 48 h only. In the combination group compared to the prednisolone group, liver arginase activity increased constantly up to 12 h returning to normal values at 48 h. Vitamin E and Se in combination may prevent the changes in arginase activity in various tissues caused by prednisolone.

Delivery of cytosolic liver arginase into the mitochondrial matrix space: a possible novel site for gene replacement therapy

As a toxic metabolic byproduct in mammals, excess ammonia is converted into urea by a series of five enzymatic reactions in the liver that constitute the urea cycle. A portion of this cycle takes place in the mitochondria, while the remainder is cytosolic. Liver arginase (L-arginine ureahydrolase, A1) is the fifth enzyme of the cycle, catalyzing the hydrolysis of arginine to ornithine and urea within the cytosol. Patients deficient in this enzyme exhibit hyperargininemia with episodic hyperammonemia and long-term effects of mental retardation and spasticity. However, the hyperammonemic effects are not so catastrophic in arginase deficiency as compared to other urea cycle defects. Earlier studies have suggested that this is due to the mitigating effect of a second isozyme of arginase (AII) expressed predominantly in the kidney and localized within the mitochondria. In order to explore the curious dual evolution of these two isozymes, and the ways in which the intriguing, aspects of AII physiology might be exploited for gene replacement therapy of AI deficiency, the cloned cDNA for human AI was inserted into an expression vector downstream from the mitochondrial targeting leader sequence for the mitochondrial enzyme ornithine transcarbamylase and transfected into a variety of recipient cell types. AI expression in the target cells was confirmed by northern blot analysis, and competition and immunoprecipitation studies showed successful translocation of the exogenous AI enzyme into the transfected cell mitochondria. Stability studies demonstrated that the translocated enzyme had a longer half-life than either native cytosolic AI or mitochondrial AII. Incubation of the transfected cells with increasing amounts of arginine produced enhanced levels of mitochondrial AI activity, a substrate-induced effect that we have previously seen with native AII but never AI. Along with exploring the basic biological questions of regulation and subcellular localization in this unique dual-enzyme system, these results suggest that the mitochondrial matrix space may be a preferred site for delivery of enzymes in gene replacement therapy.

Regulation of mRNA levels for five urea cycle enzymes in rat liver by diet, cyclic AMP, and glucocorticoids

Adaptive changes in levels of urea cycle enzymes are largely coordinate in both direction and magnitude. In order to determine the extent to which these adaptive responses reflect coordinate regulatory events at the pretranslational level, measurements of hybridizable mRNA levels for all five urea cycle enzymes were carried out for rats subjected to various dietary regimens and hormone treatments. Changes in relative abundance of the mRNAs in rats with varying dietary protein intakes are comparable to reported changes in enzyme activities, indicating that the major response to diet occurs at the pretranslational level for all five enzymes and that this response is largely coordinate. In contrast to the dietary changes, variable responses of mRNA levels were observed following intraperitoneal injections of dibutyryl cAMP and dexamethasone. mRNAs for only three urea cycle enzymes increased in response to dexamethasone. Levels of all five mRNAs increased severalfold in response to dibutyryl cAMP at both 1 and 5 h after injection, except for ornithine transcarbamylase mRNA which showed a response at 1 h but no response at 5 h. Combined effects of dexamethasone and dibutyryl cAMP were additive for only two urea cycle enzyme mRNAs, suggesting independent regulatory pathways for these two hormones. Transcription run-on assays revealed that transcription of at least two of the urea cycle enzyme genes--carbamylphosphate synthetase I and argininosuccinate synthetase--is stimulated approximately four- to fivefold by dibutyryl cAMP within 30 min. The varied hormonal responses indicate that regulatory mechanisms for modulating enzyme concentration are not identical for each of the enzymes in the pathway.