Thiazolidine derivatives as source of free L-cysteine in rat tissue.

The present study demonstrates that a variety of thiazolidine-4-(R)-carboxylic acids (TDs) which are the products of reactions of L-cysteine (cys) with carbonyl compounds could serve as a "delivery" system for cys to the cell. Liberation of the amino acid can occur enzymatically as well as non-enzymatically. The two possibilities have been proven by identification of representative compounds. The most specific substrate for mitochondrial enzymatic oxidation was thiazolidine-4-carboxylic acid (CF), the product of the reaction of cys with formaldehyde, and the least metabolized TD was 2-methyl-thiazolidine-4-carboxylic acid (CA), the product of the reaction of cys with acetaldehyde. TDs formed from cys and different sugars were not metabolized at all in mitochondria. N-Formyl-L-cysteine (NFC) the intermediate product of mitochondrial metabolism of CF was ascertained by 1H-NMR spectroscopy whereas N-acetyl-L-cysteine (NAC), the predicted metabolite of CA, was not detected, possibly due to a fast turnover. The further enzymatic hydrolysis of NFC as well as NAC to free cys was demonstrated to take place in the cytoplasm. Non-enzymatic hydrolysis of TDs depended on the chemical nature of the substituents in the thiazolidine (Th) ring. The most stable compound was unsubstituted Th and the least stable were CGlu(D) and CA. Following non-enzymatic ring opening and hydrolysis, CA was converted to methyl-djenkolic acid, which equilibrates with CA. We have identified this new compound by 1H-NMR spectroscopy. TDs may cause both a decrease and an increase in the levels of SH-groups in mitochondria. In the case of the stable CF, which is metabolized only enzymatically, an increase in the levels of SH-groups in mitochondria was observed. This suggests that enzymatic control of the breakdown of TDs prevents overflowing of the cell with thiol groups. The latter seems to be induced by high concentrations of those TDs which are hydrolysed non-enzymatically. This process leads finally to a decrease in free SH-groups by different mechanisms. The findings demonstrate two different mechanisms by which TDs can provide cys to the cells. The biological and pharmacological consequences are discussed.