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

1. Twenty-eight male rats of initial age 27 d were fed on fortified-barley diets for 3 weeks. In all experimental diets, both crude protein (nitrogen x 6.25) and methionine:cystine were constant at 120.0 g/kg dry matter (DM) and 2:1 respectively. The basal diet contained 4.5 g methionine plus cystine/kg DM with L-methionine plus L-cystine (2:1, w/w) added in increments of 0.5 g/kg DM to a final level of 7.0 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 4.2.1.22) and N5-methyltetrahydrofolate-homocysteine-methyltransferase (EC 2.1.1.13) were determined. 3. Weight gain, food consumption, FCE and carcass composition measurements of rats showed either small or no differences between the experimental diets containing 4.5--7.0 g methionine plus cystine/kg DM. For the over-all period, weight gain and FCE of rats receiving the 'positive control' diet were significantly higher than values obtained with rats receiving any of the experimental diets. 4. Cystathionine synthase activity (mumol/mg protein per 60 min; units) increased from 13.38 at 4.5 g dietary methionine plus cystine/kg DM to 18.81 at 5.0 g dietary methionine plus cystine/kg DM. The activity was then inhibited to reach a minimum value of 10.16 units at the 6.0 g/kg DM dietary level. Thereafter the activity increased to a value of 30.00 units at 7.0 g dietary methionine plus cystine/kg DM. 5. The activity of N5-methyltetrahydrofolate-methyltransferase was constant at 0.70--0.74 nmol/mg protein per 60 min between dietary levels of 4.5 and 5.0 g methionine plus cystine/kg DM. The activity then increased to a maximum value of 2.32 nmol/mg protein per 60 min at the 6.0 g/kg DM level. Thereafter the activity decreased, reaching a minimum value of 0.70 nmol/mg protein per 60 min at the 7.0 g methionine plus cystine/kg level. 6. Urinary urea-N excretion decreased significantly from 1.07 g/kg DM intake at the 4.5 g dietary methionine plus cystine/kg DM level to 1.05 g/kg DM at the 5.0 g/kg dietary level, then dropped significantly to a level of 1.01--1.00 g/kg DM intake for the higher levels of dietary methionine plus cystine.