Developmental changes in the metabolism of 3-methylthiopropionate in the rat

The oxidation of 3-methylthiopropionate (MTP), a product in the transamination pathway of methionine metabolism, was measured in rats from 1 to 400 days of age. The metabolic capacity of liver homogenates to produce volatile sulfur compounds (methanethiol and/or hydrogen sulfide) from [35S]MTP increased sixfold during the first week of life, remained at that level through weaning, then gradually decreased to essentially the value observed in the 1-day-old rat by 400 days of age. This pattern is not altered when the data are expressed in relation to tissue O2 consumption. This implies that the greater ability of young rats to produce volatile sulfur compounds is not simply a reflection of their greater metabolic rate (kilocalorie/kilogram per day). In contrast to experiments in vitro, the ability to convert a tracer dose of [35S]MTP to urinary sulfate increased from 50% of the dose in the 15-day-old rat to 80% in the 24-, 50- and 400-day-old rats. This difference between the ability to produce volatile sulfur compounds and to excrete them as sulfate may explain the toxicity of methionine in young animals.