Use of sulfate production as a measure of short-term sulfur amino acid catabolism in humans

There is no fully satisfactory method for measuring amino acid catabolism i n the nonsteady state that follows normal protein consumption. Because sulf ate is the major product of sulfur amino acid catabolism, we tested whether its production can be accurately depicted using simple tracer or nontracer approaches under basal conditions and after the intravenous administration of a known amount of sulfate. In the basal postabsorptive state, serum sul fate concentration and urinary sulfate excretion remained constant for many hours, but the apparent steady-state serum sulfate rate of appearance achi eved with primed continuous oral administration of sodium [S-34]sulfate was 20% higher than urinary sulfate excretion. By contrast, after magnesium su lfate infusion, the increase in sulfate production above basal accounted fo r 95% over 6 h and 98% over 9 h of the administered dose when measured simp ly as urinary inorganic sulfate excretion corrected for changes in its extr acellular fluid content. Using the latter method, we measured sulfate produ ction after oral methionine and intravenous infusion of methionine in a mix ture of other essential amino acids. Sulfate production above basal account ed for 59% over 6 h and 75% over 9 h of the oral methionine dose. Similar r esults were obtained with the mixed amino acid infusion, but interpretation of the latter experiment was limited by the mild protein sparing (and, hen ce, reduced endogenous sulfate production) induced by the amino acid infusi on. We conclude that a simple nontracer method can provide an accurate meas ure of sulfate production and, hence, sulfur amino acid catabolism over col lection periods as short as 6 h after a test meal. A significant portion of the sulfur derived from methionine appears to be retained in nonprotein co mpounds immediately after its ingestion.