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

Abstract

I. 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 1200 g/kg dry matter (DM) and 2: I respectively. The basal diet contained 4 5 g methionine plus cystine/kg DM with Lmethionine plus L-cystine (2: I, w/w) added in increments of 0.5 g/kg DM to a final level of 70 g methionine plus cystine/kg DM. A 'positivecontrol' diet of barley plus 193.7 g soya-bean meal/kg DM contained 6.0 g methionine plus cystine/kg DM. 3. Weight gain, food consumption, FCE and carcass composition measurements of rats showed either small or no differences between the experimental diets containing 45-7'0 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 (pmol/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 1016 units at the 6.0 g/kg DM dietary level. Thereafter the activity increased to a value of 3000 units at 7.0 g dietary methionine plus cystine/kg DM.5. The activity of N6-methyltetrahydrofolate-homocysteine-methyltransfe~ was constant at 0.704'74 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/mgprotein per 60 min at the 6.0 g/kg DM level. Thereafter the activity decreased, reaching a minimum value of 070nmol/mg protein per 60 min at the 7-0 g methionine plus cystine/kg level.6. Urinary urea-N excretion decreased significantly from I .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 I -O I -I~O g/kg DM intake for the higher levels of dietary methionine plus cystine.Results of a previous experiment (Ngwira & Beames, 1978) indicated that the requirement of dietary methionine plus cystine for optimal growth of weanling rats was 4-5-5.0 g/kg (dry matter (DM) basis), when the cystine level ,was held constant at 2.0 g/kg DM. The experiment also showed that the activities of liver cystathionine synthase (EC 4.2. I. 22) and N6-methyltetrahydrofolate-homocysteine-methyltransfera~ (mTHF Enz; EC 2. I . I . I 3)were constant between the dietary methionine plus cystine levels of 3.5 and 5-0 g/kg DM.The activities of both enzymes indicated metabolic disturbances in methionine metabolism when dietary methionine plus cysteine was increased to levels higher than 5.0 g/kg DM.Byington et al. (1972) showed that the growth of young rats was optimal when methionine: cystine was 70:30 but not when the value was less than 50:50. Finkelstein (1967) and Shannon et al. (1972) found the activity of cystathionine synthase to be inh...