Interaction between murinespf-ashmutation and genetic background yields different metabolic phenotypes

Abstract

Marini JC, Erez A, Castillo L, Lee B. Interaction between murine spf-ash mutation and genetic background yields different metabolic phenotypes. Am J Physiol Endocrinol Metab 293: E1764-E1771, 2007. First published October 9, 2007; doi:10.1152/ajpendo.00525.2007.-The spf-ash mutation in mice results in reduced hepatic and intestinal ornithine transcarbamylase. However, a reduction in enzyme activity only translates in reduced ureagenesis and hyperammonemia when an unbalanced nitrogen load is imposed. Six-week-old wild-type control and spf-ash mutant male mice from different genetic backgrounds (B6 and ICR) were infused intravenously with and L-[ring-D5]phenylalanine to investigate the interaction between genetic background and spf-ash mutation on ureagenesis, arginine metabolism, and nitric oxide production. ICR spf-ash mice maintained ureagenesis (5.5 Ϯ 0.3 mmol ⅐ kg Ϫ1 ⅐ h Ϫ1 ) and developed mild hyperammonemia (145 Ϯ 19 mol/l) when an unbalanced nitrogen load was imposed; however, B6spf-ash mice became hyperammonemic (671 Ϯ 15 mol/l) due to compromised ureagenesis (3.4 Ϯ 0.1 mmol ⅐ kg Ϫ1 ⅐ h Ϫ1 ). Ornithine supplementation restored ureagenesis and mitigated hyperammonemia. A reduction in citrulline entry rate was observed due to the mutation in both genetic backgrounds (wild-type: 128, spf-ash: 60; SE 4.0 mol ⅐ kg Ϫ1 ⅐ h Ϫ1 ). Arginine entry rate was only reduced in B6spf-ash mice (B6 spf-ash : 332, ICR spf-ash : 453; SE 20.6 mol ⅐ kg Ϫ1 ⅐ h Ϫ1 ). Genetic background and mutation had an effect on nitric oxide production (B6: 3.4, B6 spf-ash : 2.8, ICR: 9.0, ICR spf-ash : 4.6, SE 0.7 mol ⅐ kg Ϫ1 ⅐ h Ϫ1 ). Protein breakdown was the main source of arginine during the postabsorptive state and was higher in ICR spf-ash than in B6 spf-ash mice (phenylalanine entry rate 479 and 327, respectively; SE 18 mol ⅐ kg Ϫ1 ⅐ h Ϫ1 ). Our results highlight the importance of the interaction between mutation and genetic background on ureagenesis, arginine metabolism, and nitric oxide production. These observations help explain the wide phenotypic variation of ornithine transcarbamylase deficiency in the human population.arginine; nitric oxide; urea cycle ORNITHINE TRANSCARBAMYLASE (OTC) deficiency is the most common urea cycle disorder in humans (3). The reduction in ureagenesis capacity results in high plasma ammonia concentrations and frequent hyperammonemic crises that can result in coma and even death (1). OTC is also expressed in enterocytes where it functions in the synthesis of citrulline, which is then exported into the blood, serving as the precursor for arginine synthesis by the kidney (7). The metabolism of arginine (Fig. 1) is highly compartmentalized, not only involving different organs, but different intracellular compartments and subcompartments, such as mitochondria, caveolas, and cytosol (8, 33). Furthermore, different isoforms of some of the enzymes involved [e.g., arginase, nitric oxide (NO) synthase] exist or different tissue-specific subcellular localization of the same isoform may occur (6). This compartmenta...

Dietary Arginine Requirements for Growth Are Dependent on the Rate of Citrulline Production in Mice

“…The difference in the production of citrulline by the 2 genotypes may explain these findings because it can account for enough arginine to support ;0.20 g weight gain/d (see Appendix A). We previously reported similar genetic background differences in the production of citrulline between ICR and mixed-background mice (B6EiC3sn, the product of a C57BL/6J subline, and C3H/ HeSnJX) (23). A similar difference in plasma citrulline and citrulline production between C57BL/6J and FVB mice (an inbreed Swiss-derived strain) has been reported by others (24).…”

Reduced citrulline availability by OTC deficiency in mice is related to reduced nitric oxide production

“…No genotype differences in cytokine levels have been detected in response to LPS in OTC-deficient spf ash mice (27). In line with previous observations (28,29) in OTC-deficient spf ash mice of the B6 genetic background, we did not observe reduced ureagenesis in our model. Moreover, more pronounced effects of the genotype defect were present in the male mice on plasma glutamine levels and de novo arginine production, related to the X-linked genetic defect.…”

“…Subsequently, ϳ83% of the citrulline released from the small intestine into the circulation is taken up by the kidney where it is largely (ϳ75%) converted to arginine (45,49). The importance of enteral citrulline production for arginine synthesis was previously demonstrated in rats by OTC inhibition (23), in mice with the spf ash mutation causing reduced OTC activity (28), and in rats with short bowel syndrome in which both intestinal citrulline production and renal arginine production were reduced by 50% (10). Besides the intestinal-renal axis for citrulline and arginine production, an additional amount of citrulline comes from nonintestinal sources.…”

Glutamine: precursor or nitrogen donor for citrulline synthesis?

“…This means that arginine escaping first-pass extraction and metabolized somewhere else in the body to ornithine could have returned to the small intestine to be substrate for citrulline synthesis. In fact, we have previously seen substantial incorporation of plasma ornithine into citrulline (25).…”

“…It is not clear whether these human data are applicable to the other species or to humans in more physiological conditions. Nevertheless, the kinetic model employed estimates the contribution of the different precursors to citrulline appearing in the peripheral circulation (R a Cit ), which is the variable usually measured (4,23,25). Also, note that the small dilution in citrulline enrichment, due to citrulline production by nitric oxide synthase (and other minor sources), is compensated for by the increase in rate of appearance from the same source.…”