Sources of ammonia for mammalian urea synthesis

Abstract

The initial rate of incorporation of ['5N]alanine into the 6-amino group of the adenine nucleotides in rat hepatocytes was about one-eighteenth of the rate of incorporation into urea. Thus the purine nucleotide cycle cannot provide most of the ammonia needed in urea synthesis for the carbamoyl phosphate synthase reaction (EC 2.7.2.5). On the other hand, contrary to the view expressed by , because experiments on isolated mitochondria suggested that the activity of the glutamate dehydrogenase system in situ is too low because of the slow entry of glutamate into the mitochondrial matrix. These workers also noted that leucine accelerates glutamate dehydrogenase in sonicated mitochondria, whereas it inhibits urea synthesis in intact hepatocytes. These observations led to the tentative conclusion that glutamate dehydrogenase plays no major role in connection with the synthesis of urea. As for an alternative source of ammonia, they expressed the view that the deamination of AMP and the purine nucleotide cycle (Raggi et al., 1969; Lowenstein, 1972) For the measurement of the incorporation of 15N into the 6-amino group of the adenine nucleotides and into urea it is necessary to isolate a derivative from which gaseous nitrogen can readily be prepared for direct isotopic analysis in a mass spectrometer. A suitable derivative is ammonia. This was obtained from urea with urease (EC 3.5.1.5) and from adenine nucleotides by the combined action of potato apyrase (EC 3.6.1.5) and AMP deaminase (EC 3.5.4.6). In the early attempts to isolate ammonia irregular results were obtained, which were traced to the contamination of the samples with external ammonia. Such contamination dilutes adenine nucleotide N to a much greater extent than urea N (on a percentage basis) because the amounts of urea N are much greater under the test conditions. The results became consistent only when all sources of ammonia contamination had been minimized. The distilled water used was freshly prepared and then passed through a mixed-bed-resin deionizer. The HCI solutions used in the Conway units were prepared immediately before use from a BDH volumetric solution ampoule. This solution contains HgCl2 and therefore could be used only in the Conway dishes. An alternative source of low-ammonia HCI, used for neutralization of solutions to which enzymes had to be added, was the 5 M-HCI analytical volumetric

Voluntary exercise under a food restriction condition decreases blood branched-chain amino acid levels, in addition to improvement of glucose and lipid metabolism, in db mice, animal model of type 2 diabetes

“…The enlarged skeletal muscle by VE was consistent with the previous report [23]. In exercising db mice, the decrease in arginine and ornithine through possible incorporation into the liver, intermediates of the urea cycle, suggests an important role in ammonia detoxification [34]. Proline and alanine (vs. Se-db mice, P = 0.06), which decrease with exercise, may be utilized efficiently to form glucose in the liver [35].…”

Interorgan ammonia and amino acid metabolism in metabolically stable patients with cirrhosis and a TIPSS

“…Urea synthesis was strongly related with hepatic uptake of glutamine and also alanine, in agreement with in vitro observations. 36,37 1 82 48 63 111 56 2 94 312 68 380 18 3 9 7 1 4 5 1 6 5 7 8 4 71 87 38 125 30 5 96 247 68 315 22 rior mesenteric flow was 40% of portal flow in healthy volunteers. If we use these estimates, the spleen (345 nmol/kg body weight/min) and the superior mesenteric vein draining viscera (325 nmol/kg body weight/min) would produce together approximately 80% of hepatic urea production.…”

Prominent glutamine oxidation activity in mitochondria of avian transplantable hepatoma induced by MC‐29 virus

“…Glutamate dehydrogenase has high activity in the mitochondria1 matrix, and the enzyme is commonly used as a marker for the matrix space (Smith et al, 1976). The prominent role of this enzyme in ammonia genesis was reported by Krebs et al (1978). On the other hand, glutamate-ammonia ligase activity is high in the mitochondria of liver from uricotelic species (Vorhaben and Campbell, 1972).…”