Caffeine does not bind covalently to liver microsomes from different animal species and to proteins and DNA from perfused rat liver.

Caffeine was found not to bind covalently to liver microsomal proteins from mice, rats and rabbits. Microsomes metabolized caffeine only to a limited extent, the highest rate (about 2% of the substrate concentration) being obtained with rabbit microsomal preparations. The rat liver perfusion technique represents a good model for in vitro caffeine biotransformation studies and therefore for covalent binding experiments. After 2 h perfusion caffeine was extensively metabolized mainly to dimethyl and monomethyl xanthines, a minor pathway to 1,3,7-trimethyluric acid was also seen. However, covalent binding studies using the liver perfusion technique did not reveal any appreciable amount of caffeine metabolites irreversibly bound to either microsomal and total proteins and to DNA.