Quantitative study in vivo of methionine cycle in humans using [methyl-2H3]- and [1-13C]methionine

Abstract

Kinetic aspects of body methionine (MET) metabolism were examined in healthy young men during the fed and postabsorptive (PA) states. Rates of MET incorporation (S) into and release (B) from body proteins; transmethylation (TM); and remethylation (RM) and transsulfuration (TS) of homocysteine (HCY) were estimated with the aid of a 5-h constant intravenous infusion of [methyl-2H3]- and [1-13C]methionine. The isotopic data (plasma methionine labeling and 13C enrichment of expired air) were submitted to a stochastic model of amino acid metabolism. During the fed state, the subjects (n = 4) received, at 20-min intervals, small isonitrogenous isocaloric meals containing a complete L-amino acid mixture supplying MET at a rate equivalent to 198 mumol.kg body wt-1.day-1. The PA subjects (n = 4) received the isotope after a 10-h overnight fast. For the PA group, the components of MET metabolism were as follows: S, 20 +/- 0.5; B, 24 +/- 0.5; TM, 5.8 +/- 0.6; RM, 1.8 +/- 0.4; and TS, 4.0 +/- 0.4 (+/-SE) mumol.kg-1.h-1. During the fed state the values were S, 26 +/- 2.5; B, 18 +/- 2; TM, 14 +/- 1.3; RM, 5.7 +/- 0.9; and TS 8.3 +/- 0.6 mumol.kg-1.h-1. The meal-induced changes in B, TM, RM, and TS were significant (P less than 0.05). Comparison of the partitioning of MET between S and TM (these two pathways of MET disposal constitute the "methionine locus") in the PA and in the fed states indicates that the MET locus is of regulatory importance in MET homeostasis. A twofold increase in the partitioning of MET to TM was observed in the fed state. The increase in HCY recycling, relative to TS (these two pathways of HCY disposal constitute the "HCY locus"), in the fed state did not reach statistical significance when compared with the PA state. Total daily TM are estimated to be 238 +/- 22 mumol/kg. This is similar to the estimate generated by the methyl balance model of Mudd and Poole (Metabolism 24: 721, 1975) which approximated 241 mumol/kg.

Effects ofdl-2-hydroxy-4-methylthiobutyrate on the first-pass intestinal metabolism of dietary methionine and its extra-intestinal availability

“…Although the similar portal mass balance combined with the relatively lower input of directly available L-methionine in the DL-HMTB than in the DL-MET diet may in part account for the difference in the fractional portal balance between the treatments, it did not exclude the possibility that the DL-HMTB diet had a lower first-pass metabolism of methionine by the intestine than the DL-MET diet. This notion was supported by the evidence that the trans-sulfuration reaction of methionine in splanchnic tissues was affected by dietary methionine levels (20) . Because methionine converted from the sources was not included in the calculated methionine input, which may result in overestimate of the fractional portal balance of dietary methionine, the stable isotope tracer methionine ([1-13 C]methionine) was used in the present study to further estimate the proportion of L-methionine that was utilised in the first pass by the intestine.…”

Remethylation and transsulfuration of methionine in cirrhosis: Studies with l-[2H3-methyl-1-13C]methionine

“…Statistically significant differences were observed for vitamin B 12 and aspartate aminotransferase, which were both higher in patients with cirrhosis as compared with healthy volunteers. Patients with cirrhosis expired less CO 2 than controls and had a higher body weight, although only 1 patient had moderate ascites.…”

“…Total HCY in plasma was measured by high-performance liquid chromatography after reduction of disulfides and derivatization of thiols with monobromobimane as previously described. 15 Determination of Enzyme Activities, Folate, and Vitamin B 12 . Vitamin B 12 , folate, and the activities of alanine amino transferase and aspartate aminotransferase were determined by routine methods in use in the university hospital's central chemical laboratory.…”

“…The isotopic labeling of the carboxyl and the methyl group allows us to follow transsulfuration by the decarboxylation to 13 CO 2 and remethylation by assessing the replacement of the labeled methyl group by an unlabeled one. This approach using either continuously infused or orally administered tracer has been previously used to investigate MET metabolism in healthy subjects [9][10][11][12] and patients with end-stage renal disease. 13 …”

Suplementação De Creatina, Homocisteína E Estresse Oxidativo

“…Wyss e Kaddurah-Daouk 1 relataram que esse processo utiliza mais S-adenosilmetionina (SAM) que todas as outras reações de metiltransferases juntas. Storck et al 38 em seu estudo pioneiro utilizado a téc-nica de isótopos estáveis para estimar o balanço de reações SAM, encontraram uma taxa de transmetilação de 16,6 mmol/l, resultados condizentes com os achados de Mudd e Poole 36 que encontraram uma taxa variante de 14 a 18 mmol/l, confirmando a teoria proposta por esses autores de que a síntese endógena de creatina é o maior modulador do balanço de metilação e formação da Hcy.…”