Growth and liver enzyme response in growing rats to graded levels of methionine and cystine in fortified-barley diets: response at constant cystine concentration in the diet.

I . Thirty-six male rats of initial age 27 d were fed on diets based on barley grain for 3 weeks. Crude protein (nitrogen x 6.25) was fixed at 120.0 g/kg dry matter (DM) and cystine at 2.0 g/kg DM in all diets. The basal diet contained 3 3 g methionine plus cystine/kg DM with L-methionine added in increments of 0.5 g/kg DM up to a final level of 7.0 g methionine plus cystine/kg DM. A ‘positive control’ diet of barley plus 193.7 g soya-bean meal/kg DM contained 6.0 g methionine plus cystine/kg DM. 2. Weight gain, food conversion efficiency (FCE), urinary urea-N excretion, carcass composition and activities of liver cystathionine synthase (EC 4 . 2 . I . 22) and N5-methyltetrahydrofolate-homocysteinemethyltransferase (EC 2 . I . I . 13) were determined. 3. Weight gain, FCE and urinary urea-N excretion indicated that levels of 3 3 and 4.0 g methionine plus cystine/kg DM were inadequate for optimal growth of the rats. These variables indicated 4.7 g/kg DM to be the requirement for methionine plus cystine. Carcass composition results gave no clear indication of optimal requirements for methionine plus cystine. 4. Cystathionine synthase and N5-methyltetrahydrofolate-homocysteine-methyltransferase activities (/mg protein per 60 min) were constant at 24.32 pmol and 0 6 4 nmol respectively, when methionine plus cystine level was between 3 3 and 5.0 g/kg DM. Cystathionine synthase was then inhibited at the 6.0 g/kg DM level to a minimum activity of 16.12 pmol/mg protein per 60 min, whereas activity of the re-methylation enzyme increased to a maximum level of 2.92 nmol/mg protein per 60 min at the 6.0 g/kg DM level of methionine plus cystine. At the levels of methionine plus cystine above 6.0 g/kg DM cystathionine synthase activity increased while the activity of Ns-methyltetrahydrofolate-homocysteine-methyltransferase decreased.