<Tr> <Td> </Td> <Td> </Td> </Tr> <P> Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells . The compound is a dinucleotide, because it consists of two nucleotides joined through their phosphate groups . One nucleotide contains an adenine base and the other nicotinamide . Nicotinamide adenine dinucleotide exists in two forms, an oxidized and reduced form abbreviated as NAD and NADH respectively . </P> <P> In metabolism, nicotinamide adenine dinucleotide is involved in redox reactions, carrying electrons from one reaction to another . The coenzyme is, therefore, found in two forms in cells: NAD is an oxidizing agent--it accepts electrons from other molecules and becomes reduced . This reaction forms NADH, which can then be used as a reducing agent to donate electrons . These electron transfer reactions are the main function of NAD . However, it is also used in other cellular processes, the most notable one being a substrate of enzymes that add or remove chemical groups from proteins, in posttranslational modifications . Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery . </P> <P> In organisms, NAD can be synthesized from simple building - blocks (de novo) from the amino acids tryptophan or aspartic acid . In an alternative fashion, more complex components of the coenzymes are taken up from food as the vitamin called niacin . Similar compounds are released by reactions that break down the structure of NAD . These preformed components then pass through a salvage pathway that recycles them back into the active form . Some NAD is also converted into nicotinamide adenine dinucleotide phosphate (NADP); the chemistry of this related coenzyme is similar to that of NAD, but it has different roles in metabolism . </P>

What is the difference between nad and nadh
find me the text answering this question