Catalyzes the NADPH-dependent reduction of carbonyl compounds to their corresponding alcohols. Has low NADPH-dependent oxidoreductase activity. Acts on several orthoquinones, acts as well on non-quinone compounds, such as isatin or on the anticancer drug oracin. Best substrates for CBR3 is 1,2- naphthoquinone, hence could play a role in protection against cytotoxicity of exogenous quinones. Exerts activity toward ortho-quinones but not paraquinones. No endogenous substrate for CBR3 except isatin has been identified. a secondary alcohol + NADP(+) = a ketone + H(+) + NADPH a quinone + H(+) + NADPH = a quinol + NADP(+) Belongs to the short-chain dehydrogenases/reductases (SDR) family.