Localization and capacity of the last step of mercapturic acid biosynthesis and the reabsorption and acetylation of cysteine S-conjugates in the rat kidney

Abstract

We investigated the capacity and the localization of N-acetylation of the mercapturic acid precursor S-benzyl-l-cysteine (BC), as well as the tubular reabsorption of this compound in the rat kidney in vivo et situ by renal clearance and continuous microinfusion and microperfusion experiments. In renal clearance experiments, 450 μmol BC was infused intravenously for 180 min. During the time of BC infusion and the following 180 min, the two kidneys excreted 400 μmol or 90% of the infused BC dose as the mercapturate N-acetyl-S-benzyl-l-cysteine (AcBC). Comparison of the amounts of BC and AcBC entering the left kidney via the renal artery with those leaving it via the renal vein and the ureter showed that 0.13±0.04 μmol BC/min (mean ± SEM) was extracted and 0.24±0.08 μmol AcBC/min was formed by one kidney. The intrarenal acetylation can account for the formation of 38% of the mercapturate excreted in the final urine. In additional experiments, 50 pmol/min [¹⁴C]BC was microinfused into single superficial tubules at three different sites. During microinfusion into early proximal tubules, the final urine contained 16.3±1.8% of the microinfused radioactivity as AcBC, but no BC. When [¹⁴C]BC was microinfused into late proximal tubules, 13.0±2.3% of the infused label was recovered as BC, 28.1±2.3% as AcBC. During microinfusion into early distal tubules, the final urine contained no AcBC, but 90.3±2.1% of the infused [¹⁴C]BC was recovered. As the infused BC dose was reabsorbed completely by the high-capacity carrier for neutral amino acids in the proximal convolution, our results show acetylation of the cysteine S-conjugate both by the convoluted and the straight part of the proximal nephron, the capacity for acetylation and/or secretion being almost twice as high in the straight part.