Formation of Thioproline, Effective as a Nitrite-Trapping Agent in the Human Body, in Various Cooked Foods

Abstract

Thioproline (TPRO, thiazolidine-4-carboxylic acid) and methylthioproline (MTPRO) are the condensation products of cysteine with formaldehyde and acetaldehyde, respectively. TPRO and MTPRO are thought to be "frozen" cys-teines because they liberate cysteine in the liver and protect the liver. We identified N-nitrosothioproline (NTPRO) and N-nitrosomethylthioprolines (cis and trans) as the major N-nitroso compounds (NOC) in human urine. NTPRO is a nonmutagenic NOC, and thus is probably noncarcinogenic, like N-nitrosoproline. Nitrosation ofTPRO is nearly 1000 times faster than that of proline. NTPRO is excreted into urine without further metabolie change, and has been used as a sensitive monitoring probe like N-nitrosoproline for endogenous formation of NOC. TPRO and MTPRO may play an important role in cancer prevention due to their effective nitrite-trapping capacity in the human body. The anticarcinogenic effect ofTPRO has experimentally demonstrated by inhibition of stomach cancer induced by coadministration of benzylmethylamine and nitrite in rats. TPRO was ubiquitously found in various cooked foods, such as vegetables (~2ppm), cod (~5ppm), shiitake mushrooms (~10 ppm), and liver-based foods (~15 ppm). Especially high contents were detected in djenkol beans (~800 ppm), wh ich are eaten in Indonesia, and several types of leguminous beans (>3500 ppm) eaten in southern Thailand.