Changes in body-weight, composition and hepatic enzyme activities in response to dietary methionine, betaine and choline levels in growing chicks.

The experiments described here were set up (a) to investigate the effect of age and (b) to investigate the effect of giving five diets which varied in methionine and choline or betaine contents on some of the enzymes that metabolize these nutrients in chick liver. Growth and carcass composition of the chicks fed on the different diets were also examined. There was no obvious relationship between age and enzyme activity in young chicks. Only a diet low in methionine (but not one low in choline) showed a significant decrease in growth and a change in carcass composition. The effects of diet on enzyme activity were complex. Choline oxidase (EC 1.1.3.17) activity was affected by the level of choline in the diet, being high when choline was present at high levels, especially when methionine was limiting. 5-Methyltetrahydrofolate homocysteine methyltransferase (EC 2.1.1.3) had a high activity in the livers of chicks fed on a conventional diet compared with those given semi-purified diets. Other enzymes showed minor changes in response to the diet. The diet low in methionine showed a lower activity of cystathionine beta-synthase (EC 4.2.1.22) and slightly higher activities of methionine adenosyltransferase (EC 2.5.1.6) and betaine-homocysteine methyltransferase (EC 2.1.1.5; compared with other diets), suggesting that this diet encouraged re-methylation of homocysteine at the expense of trans-sulphuration to cystathionine. The findings obtained in these studies form a useful basis for further investigation of the metabolic interrelationships between methionine and related nutrients.