title: Rapid stimulation of the hepatic glycine-cleavage system in rats fed on a single high-protein meal.
abstract: Glycine catabolism was studied in isolated rat liver mitochondria by measuring the release of 14CO2 from [1-14C]glycine. Mitochondria isolated from rats fed on a high-protein (60% casein) diet for 5 days showed an enhanced ability to catabolize glycine compared with mitochondria from rats fed on a normal-protein (15% casein) diet. Glycine catabolism was also stimulated in normal protein-fed rats if they ingested a single high-protein meal for 2 h before being killed, thus illustrating the rapid response of the glycine-cleavage system to protein intake. The stimulation of glycine catabolism in rats given a high-protein diet or meal was not evident if the mitochondria were incubated in the absence of P(i) (omitting ADP had no effect on the rate). Mitochondria from high-protein- and normal-protein-fed rats did not differ in their ability to accumulate glycine, a process which occurred far too rapidly to impose a limit on the rate of flux through the glycine-cleavage system. The stimulation of glycine catabolism by high-protein feeding was not associated with a change in mitochondrial matrix volume. Furthermore, mitochondria from rats fed on a high-protein meal maintained an enhanced ability to catabolize glycine compared with those from rats fed on a normal-protein meal when incubated in hypo-osmotic solutions of very low osmolarity. When mitochondria from high-protein- or normal-protein-fed rats were maximally activated by incubation in the presence of 0.25 microM-Ca2+, the rates of glycine catabolism were high, but similar, showing that the stimulation of glycine catabolism by high-protein feeding does not involve an increase in the total capacity of the system. These findings show that hepatic glycine catabolism is stimulated rapidly by high-protein feeding, a response that we suggest is involved in the disposal of the excess glycine in the diet.
authors.affiliation: Department of Biochemistry, Memorial University of Newfoundland, St. John's, Canada.
authors.forename: H S
authors.initials: HS
authors.lastname: Ewart
authors.forename: M
authors.initials: M
authors.lastname: Jois
authors.forename: J T
authors.initials: JT
authors.lastname: Brosnan
mesh_terms: D000594:Amino Acid Oxidoreductases / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D004044:Dietary Proteins / Q000494:pharmacology*; D005998:Glycine / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008930:Mitochondria, Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012016:Reference Values; D014166:Transferases / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D004044:Dietary Proteins; D009097:Multienzyme Complexes; C064525:glycine cleavage system; D000594:Amino Acid Oxidoreductases; D014166:Transferases; D005998:Glycine