Insulin in methionine and homocysteine kinetics in healthy humans: plasma vs. intracellular models.
Methionine is a sulfur-containing amino acid that is reversibly converted into homocysteine. Homocysteine is an independent cardiovascular risk factor frequently associated with the insulin resistance syndrome. The effects of insulin on methionine and homocysteine kinetics in vivo are not known. Six middle-aged male volunteers were infused with L-[methyl-2H3,1-13C]methionine before (for 3 h) and after (for 3 additional hours) an euglycemic hyperinsulinemic (150 mU/l) clamp. Steady-state methionine and homocysteine kinetics were determined using either plasma (i.e., those of methionine) or intracellular (i.e., those of plasma homocysteine) enrichments. By use of plasma enrichments, insulin decreased methionine rate of appearance (Ra; both methyl- and carbon Ra) by 25% (P < 0.003 vs. basal) and methionine disposal into proteins by 50% (P < 0.0005), whereas it increased homocysteine clearance by approximately 70% (P < 0.025). With intracellular enrichments, insulin increased all kinetic rates, mainly because homocysteine enrichment decreased by approximately 40% (P < 0.001). In particular, transmethylation increased sixfold (P < 0.02), transsulfuration fourfold (P = 0.01), remethylation eightfold (P < 0.025), and clearance eightfold (P < 0.004). In summary, 1) physiological hyperinsulinemia stimulated homocysteine metabolic clearance irrespective of the model used; and 2) divergent changes in plasma methionine and homocysteine enrichments were observed after hyperinsulinemia, resulting in different changes in methionine and homocysteine kinetics. In conclusion, insulin increases homocysteine clearance in vivo and may thus prevent homocysteine accumulation in body fluids. Use of plasma homocysteine as a surrogate of intracellular methionine enrichment, after acute perturbations such as insulin infusion, needs to be critically reassessed.
Tessari
Paolo
P
Dept. of Clinical and Experimental Medicine, Chair of Metabolism, Policlinico Universitario, via Giustiniani 2, 35128 Padua, Italy. paolo.tessari@unipd.it
Kiwanuka
Edward
E
Coracina
Anna
A
Zaramella
Michela
M
Vettore
Monica
M
Valerio
Anna
A
Garibotto
Giacomo
G
D001786:Blood Glucose / Q000378:metabolism; D001944:Breath Tests; D015309:Glucose Clamp Technique; D006710:Homocysteine / Q000097:blood* / Q000493:pharmacokinetics*; D006801:Humans; D006946:Hyperinsulinism / Q000097:blood / Q000378:metabolism*; D007328:Insulin / Q000097:blood; D007527:Isoenzymes; D007700:Kinetics; D008297:Male; D008715:Methionine / Q000097:blood* / Q000493:pharmacokinetics*; D042965:Methylenetetrahydrofolate Reductase (NADPH2) / Q000378:metabolism; D008875:Middle Aged; D011865:Radioisotope Dilution Technique
D001786:Blood Glucose; D007328:Insulin; D007527:Isoenzymes; D006710:Homocysteine; D008715:Methionine; D042965:Methylenetetrahydrofolate Reductase (NADPH2)