Fig 1.
In the urea cycle the “toxin” ammonium (NH4+) is converted to non-toxic urea by five consecutive enzymatic reactions in the liver. In hepatocytes, the rate-limiting, ATP-dependent enzyme carbamoyl phosphate synthetase 1 (CPS1), which is allosterically activated by N-acetyl glutamate (NAG), produced by N-acetyl glutamate synthase (NAGS), not shown, and ornithine transcarbamylase (OTC) are located in the mitochondria, while argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL) and arginase (ARG) are in the cytoplasm. Inherited defects in any of these enzymes can cause recurrent episodes of hyperammonemia. Defects in two mitochondrial transporters, not shown, may also result in hyperammonemia.
Table 1.
Summary of the Swiss cohort of OTC deficient patients.
Fig 2.
Overview of included patients and occurrence of acute liver failure.
From 37 cases, 29 were eligible for further studies regarding occurrence of acute liver failure (ALF). Male patients are shown as neonatal (onset in the first month of life) and late onset (symptoms beyond the first month of life) cases. Females are classified as symptomatic or asymptomatic. The bottom boxes indicate the occurrence of ALF.
Table 2.
Laboratory values of OTCD patients suffering from acute liver failure.
Table 3.
Association of coagulopathy and acute liver failure with hyperammonemia.
Table 4.
Unresponsiveness to vitamin K in coagulopathy occurring in OTCD.
Fig 3.
INR is a sensitive parameter of ammonia-related liver dysfunction.
Fig 3 illustrates laboratory values from patient 21.1 with neonatal onset of OTCD causing acute liver failure (ALF) and fatal disease course. Concomitant with massive hyperammonemia (max. 2569 μmol/L; upper left y-axis in grey colour), INR was drastically elevated (max. 4.1; lower left y-axis in red colour) while both liver transaminases (here only ALAT is shown; max. 25 U/L) remained normal. After suspicion of OTCD, this patient immediately received specific treatment including hemodialysis resulting in rapid normalization of plasma ammonia levels after two days of treatment. However, INR remained elevated despite application of fresh frozen plasma and the patient deceased from multiorgan failure.
Fig 4.
Effects of NH4Cl exposure on human primary hepatocytes.
Following treatment with NH4Cl, several parameters were determined in cell culture supernatant: A, Cell viability showed no significant differences between various treatment groups (0; 0.1; 1; 10 mM NH4Cl for 24 h) compared to non-treated control cells as assessed by MTT assay. B, Albumin secretion was quantified by ELISA. While control cells secreted 1944 ± 235 μg/L albumin in 24 hours, cells treated with 10 mM NH4Cl secreted only 1471 ± 25 μg/L and after 48 hours controls secreted 2820 ± 464 μg/L versus 1666 ± 53 μg/L in treated cells. C, Quantification of urea: while in untreated hepatocytes urea in supernatant increases slightly from 0.42 mmol/L at 24 h to 0.64 mmol/L at 48 h, in NH4Cl-treated cells urea production increases from 0.82 mmol/L to 1.31 mmol/L, respectively. D, Quantification of ASAT reveals a potential negative effect of ammonia on mitochondrial integrity.