Growth and liver enzyme response to dietary levels of sulphur amino acids in growing rats and pigs receiving barley-based diets

The growth of weanling rats receiving varying levels of methionine plus cystine in barley-based diets showed that 0.23-0.40% dry matter (DM) basis levels could not support optimal growth. No significant differences were detected in carcass composition or growth of rats receiving 0.45-0.70% DM methionine plus cystine dietary levels when either the cystine level was held constant at 0.20% DM in all diets or the methionine:cystine ratio was held constant at 2:1 and 1:1 in all diets. When either the dietary cystine concentration was held constant at 0.20% DM or the methionine:cystine ratio was held constant at 2:1 in all diets, the results indicated that cystathionine synthase activity was constant between 0.35 and 0.50% DM dietary methionine plus cystine. The activity of the enzyme was then inhibited, reaching minimum activity at the 0.60% DM dietary methionine plus cystine level. Thereafter, the activity increased to levels higher than the activity levels obtained between 0.35 and 0.50% DM dietary methionine plus cystine levels. These results indicate that 0.50% DM dietary methionine plus cystine is the limit for normal methionine metabolism. When either the dietary cystine concentration was held constant at 0.20% DM or the methionine:cystine ratio was held constant at 2:1 in all diets, the activity of N⁵-methyltetra-hydrofolate-homocysteine-methyltransferase (mTHF Enz.) was constant between 0.35 and 0.50% DM methionine plus cystine. The enzyme activity then increased, reaching maximum levels at the 0.60% DM methionine plus cystine level. Thereafter the enzyme activity was inhibited to levels corresponding to activities obtained between 0.35-0.50% DM dietary methionine plus cystine. The results indicate that normal methionine metabolism occurs up to the 0.50% DM methionine plus cystine in the diet. When the methionine:cystine ratio was held constant at 1:1 in all diets in which the methionine plus cystine concentration varied between 0.35 and 0.70% DM, cystathionine synthase activity did not respond to the varying levels of methionine plus cystine, whereas mTHF Enz. activity was inhibited progressively with increasing levels of dietary methionine plus cystine. The same results showed that increasing the dietary serine level from 0.38 to 0.58% DM depressed feed intake and the activities of both cystathionine synthase and mTHF Enz. in rats fed the 0.35-0.70% DM dietary methionine plus cystine range. The interaction between choline and methionine plus cystine when the methionine: cystine ratio was held at 1:1 in all diets showed that the 1200 mg choline chloride/kg DM diet inhibited mTHF Enz. activity and activated cystathionine synthase more than the 1000 mg choline chloride/kg DM diet. The pig trial, in which gilts were fed barley-based diets containing varying levels of methionine plus cystine, showed that the change in urinary urea-nitrogen excretion of pigs on test diets from the positive control diet, could be used as an indicator of methionine plus cystine requirements for optimal growth. Using this parameter, the requirement for methionine plus cystine for the 32.6±0.6 kg gilt was 0.55% DM on barley-based diets where cystine was held constant at 0.20% DM level.Land and Food Systems, Faculty ofGraduat