Intestinal ammonium production in the rat: the role of the colon, small intestine, and circulating glutamine

An engineered E. coli Nissle improves hyperammonemia and survival in mice and shows dose-dependent exposure in healthy humans

The intestine is a major source of systemic ammonia (NH₃); thus, capturing part of gut NH₃ may mitigate disease symptoms in conditions of hyperammonemia such as urea cycle disorders and hepatic encephalopathy. As an approach to the lowering of blood ammonia arising from the intestine, we engineered the orally delivered probiotic Escherichia coli Nissle 1917 to create strain SYNB1020 that converts NH₃ to l-arginine (l-arg). We up-regulated arginine biosynthesis in SYNB1020 by deleting a negative regulator of l-arg biosynthesis and inserting a feedback-resistant l-arg biosynthetic enzyme. SYNB1020 produced l-arg and consumed NH₃ in an in vitro system. SYNB1020 reduced systemic hyperammonemia, improved survival in ornithine transcarbamylase-deficient spf^ash mice, and decreased hyperammonemia in the thioacetamide-induced liver injury mouse model. A phase 1 clinical study was conducted including 52 male and female healthy adult volunteers. SYNB1020 was well tolerated at daily doses of up to 1.5 × 10¹² colony-forming units administered for up to 14 days. A statistically significant dose-dependent increase in urinary nitrate, plasma ¹⁵N-nitrate (highest dose versus placebo, P = 0.0015), and urinary ¹⁵N-nitrate was demonstrated, indicating in vivo SYNB1020 activity. SYNB1020 concentrations reached steady state by the second day of dosing, and excreted cells were alive and metabolically active as evidenced by fecal arginine production in response to added ammonium chloride. SYNB1020 was no longer detectable in feces 2 weeks after the last dose. These results support further clinical development of SYNB1020 for hyperammonemia disorders including urea cycle disorders and hepatic encephalopathy.

Reversal of immunodominance among autoantigenic T-cell epitopes

Studies spanning several decades have revealed how the complex forces of antigen processing distinguish those epitopes of a protein that dominate the immune response from those that remain cryptic. Since foreign antigens and self-proteins are subjected to the same proteolytic pathways before presentation to the T-cell repertoire, it has long been assumed that they comply equally with the established rules of immunodominance. Nevertheless, the pathological determinants of some autoantigens appear ill-equipped for the dominant role they adopt, displaying features more befitting subdominant or cryptic epitopes, such as low affinity for their MHC restriction element. These findings may be reconciled by suggesting that, far from remaining sequestered during ontogeny, many classical autoantigens participate in the establishment of self-tolerance, the efficiency with which individual epitopes purge the T-cell repertoire being determined by the conventional rules of immunodominance: while those epitopes that are truly dominant induce profound non-responsiveness, those that are poorly presented may leave residual reactivity, manifest in the periphery as responses to epitopes that appear inappropriately dominant. Here we review recent evidence showing the process of self-tolerance to be uniquely responsible for the reversal of immunodominance which promotes such epitopes to an undeserved position of importance within the determinant hierarchy.

Altered glutamine metabolism in rat portal drained viscera and hindquarter during hyperammonemia

In normal rats, muscle is the major glutamine releasing organ and gut is the major glutamine consuming organ. It has been suggested that enhanced muscle ammonia detoxification and gut ammonia production occurs during liver insufficiency-induced hyperammonemia. Therefore, ammonia and amino acid fluxes across portal-drained viscera and hindquarter, and muscle concentrations were measured in portacaval shunted and acute liver ischemia rats. Arterial ammonia and most amino acids were increased after portacaval shunting and increased progressively during liver ischemia, but net hindquarter ammonia uptake was not observed. Net hindquarter glutamine efflux was increased during portacaval shunting, but it decreased during liver ischemia, while muscle glutamine concentrations increased. The comparable net portal drained viscera glutamine uptake in normal and portacaval shunted rats changed during liver ischemia from net uptake to release, coinciding with release of most other amino acids. These results cast doubt on the ammonia detoxifying role of muscle during acute liver ischemia-induced hyperammonemia in the rat. The portal drained viscera glutamine release during severe hyperammonemia could be due to intestinal damage.

Ammonia elimination as a rapid index of viability in liver grafts in dogs

We examined intraoperative changes in blood ammonia levels and the correlation with graft viability in orthotopic liver transplantation (OLTx) in 29 dogs. Blood ammonia levels following total hepatectomy were examined using five dogs. These levels immediately following hepatectomy (15 min) were significantly higher (212 +/- 29 micrograms/dl) over values noted before hepatectomy (93 +/- 11 micrograms/dl, P less than 0.05). OLTx was performed using the cuff technique. The animals were divided into two groups: Group A (n = 6 pairs), OLTx with a nonpreserved fresh graft; Group B (n = 6 pairs), OLTx with an 8-hr preserved graft with lactated Ringer (4 degrees C). In both groups, the blood ammonia levels before the surgery and at the anhepatic phase data did not differ; however, following reperfusion, the levels in Group B were significantly higher (211 +/- 26 at 15 min, 200 +/- 50 micrograms/dl at 30 min) than those in Group A (121 +/- 10 at 15 min; P less than 0.01, 109 +/- 9 micrograms/dl at 30 min; P less than 0.05). The blood ammonia level highly correlated with adenosine triphosphate contents in the liver tissue, blood level of lactic acid, and amount of bile output, all pertinent indicators of the graft viability. Thus, the potential to eliminate ammonia immediately after reperfusion can serve as an indicator of graft viability. The intraoperative monitoring of blood ammonia levels can be included in management guidelines in cases of liver transplantation.