THE IMPORTANCE OF TRANSMETHYLATION REACTIONS TO METHIONINE METABOLISMIN SHEEP - EFFECTS OF SUPPLEMENTATION WITH CREATINE AND CHOLINE

The influence of administering the methylated products choline and cre atine on methionine irreversible-loss rate (ILR) and recycling from ho mocysteine has been investigated in sheep fed close to energy and N eq uilibrium. Two methods to estimate methionine recycling were compared. The first involved [U-C-13]methionine infused as part of a labelled a mino acid mixture obtained from hydrolysed algal protein. In this appr oach the isotope dilution of methionine with all five C atoms labelled (m+5) will represent the ILR which does not recycle through homocyste ine, while that which includes molecules with C-1-C-4 labelled will al low for loss of the labelled methyl (5)-C atom and replacement by an u nlabelled moiety in the remethylation of homocysteine. The second meth od involved a combined infusion of [1-C-13]- and [S-methyl-H-2(3)]meth ionine. These two approaches gave similar data for methionine ILR whic h does not include label recycled to the amino acid from homocysteine but differed for recycled methionine fluxes. Consequently the two proc edures differed in the calculated extent of homocysteine methylation u nder control conditions (6 v. 28 %). These extents of remethylation ar e within the range observed for the fed human subject, despite the fac t that fewer dietary methyl groups are available for the ruminant. Usi ng combined data from the infusions, significant depression of methion ine recycling occurred in blood (P < 0.05), with a similar trend for p lasma (P = 0.077), when choline plus creatine were infused. Wool growt h, assessed by intradermal injection of [S-35]cysteine, was not altere d by supplementation with the methylated products. From changes in the label pattern of free methionine in aortal, hepatic portal and hepati c venous blood during U-C-13-labelled algal hydrolysate infusion, the major sites of homocysteine remethylation appear to be the portal-drai ned viscera and the liver. This was confirmed by analysis of free meth ionine enrichments in various tissues following dual infusion of [1-C- 13]- and [S-methyl-H-2(3)]methionine, with the greatest activities occ urring in rumen, jejunum and liver. Of the nonsplanchnic tissues exami ned, only kidney exhibited substantial methionine cycling; none was de tected in muscle, heart, lung and skin. The implications of methyl gro up provision under net production conditions are discussed.