Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Proceedings
Publication Acceptance Date: 2/20/2000
Publication Date: 7/5/2000
Citation: LEE, B., YU, H., JAHOOR, F., O'BRIEN, W., BEAUDET, A.L., REEDS, P.J. IN VIVO UREA CYCLE FLUX DISTINGUISHES AND CORRELATES WITH PHENOTYPIC SEVERITY IN DISORDERS OF THE UREA CYCLE. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. 2000. v. 97(14). p. 8021-8026. Interpretive Summary: Excess protein that we eat is first broken down to a toxic compound, ammonia which is then converted to a harmless compound called urea and excreted from the body. However, some people are born with a genetic disorder which makes them unable to convert ammonia to urea. The ammonia builds up in their blood making them very sick and some even die. The extent to which they cannot convert ammonia to urea or the extent to which a drug is helping them to improve the conversion of ammonia to urea is hard to measure. In this study the doctors invented a new method to do this measurement very accurately.
Technical Abstract: Urea cycle disorders are a group of inborn errors of hepatic metabolism that result in often life-threatening hyperammonemia and hyperglutaminemia. Clinical and laboratory diagnosis of partial deficiencies during asymptomatic periods is difficult, and correlation of phenotypic severity with either genotype and/or in vitro enzyme activity is often imprecise. We hypothesized that stable isotopically determined in vivo rates of total body urea synthesis and urea cycle-specific nitrogen flux would correlate with both phenotypic severity and carrier status in patients with a variety of different enzymatic deficiencies of the urea cycle. We studied control subjects, patients, and their relatives with different enzymatic deficiencies affecting the urea cycle while consuming a low protein diet. On a separate occasion the subjects either received a higher protein intake or were treated with an alternative route medication sodium phenylacetate/benzoate (Ucephan), or oral arginine supplementation. Total urea synthesis from all nitrogen sources was determined from [18O]urea labeling, and the utilization of peripheral nitrogen was estimated from the relative isotopic enrichments of [15N]urea and [15N]glutamine during i.v. co-infusions of [5-(amide)15N]glutamine and [18O]urea. The ratio of the isotopic enrichments of 15N-urea/15N-glutamine distinguished normal control subjects (ratio = 0.42 ± 0.06) from urea cycle patients with late (0.17 ± 0.03) and neonatal (0.003 ± 0.007) presentations irrespective of enzymatic deficiency. This index of urea cycle activity also distinguished asymptomatic heterozygous carriers of argininosuccinate synthetase deficiency (0.22 ± 0.03), argininosuccinate lyase deficiency (0.35 ± 0.11), and partial ornithine transcarbamylase deficiency (0.26 ± 0.06) from normal controls. Administration of Ucephan lowered, and arginine increased, urea synthesis to the degree predicted from their respective rates of metabolism. The 15N-urea/15N-glutamine ratio is a sensitive index of in vivo urea cycle activity and correlates with clinical severity. Urea synthesis is altered by alternative route medications and arginine supplementation to the degree that is to be expected from theory. This stable isotope protocol provides a sensitive tool for evaluating the efficacy of therapeutic modalities and acts as an aid to the diagnosis and management of urea cycle patients.