Reduction is the reverse of oxidation and therefore it can involve loss of an oxygen atom or the addition of two hydrogen atoms. Many of the same enzymes that are involved in oxidation can also mediate reductions. For example, some drugs, especially those that are very electron deficient because of nitro groups, etc., can be reduced by cytochromes P450. Some other enzymes, such as alcohol dehydrogenase (ALD) are readily reversible and the same enzyme can also catalyze reduction. Much reduction occurs in anaerobic bacteria in the gut because, being anaerobic, much of the metabolism of these organisms involves reductive pathways.