Figure 2.

Examples of non-enzymatic synthesis of complex compounds from simpler metabolites without Schiff base involvement. See Fig. 1 for general comments. A, dopamine exemplifies any catecholamine. Quinone formation is possible without oxygen involvement, being driven by ferric iron reduction (28). Cysteine may be a free amino acid or a part of glutathione or protein. 5-S-Cysteinyl dopamine is mostly protein-bound, whereas the free cysteine gives rise to cytotoxic dihydrobenzothiazine derivatives (18, 29). B, carbamylation of any amino-group bearer is possible. Only the homocitrulline level in human skin has been shown so far to increase with age (30). C, high reactivity of thioesters is long acknowledged in chemistry. Their ability to acylate cellular proteins non-enzymatically is reported in several recent publications (20, 31, 32).