Phenylketonuria (PKU) is caused by decreased activity of phenylalanine hydroxylase (PAH), an enzyme that converts the amino acid phenylalanine to tyrosine , a precursor of several important hormones and skin, hair, and eye pigments. Decreased PAH activity results in accumulation of phenylalanine and a decreased amount of tyrosine and other metabolites. Persistent high levels of phenylalanine in the blood in turn result in progressive developmental delay, a small head circumference, behaviour disturbances, and seizures. Due to a decreased amount of the pigment melanin , persons with PKU tend to have lighter features, such as blond hair and blue eyes, than other family members who do not have the disease. Treatment with special formulas and with foods low in phenylalanine and protein can reduce phenylalanine levels to normal and maintain normal intelligence. However, rare cases of PKU that result from impaired metabolism of biopterin, an essential cofactor in the phenylalanine hydroxylase reaction, may not consistently respond to therapy.
Type 2 diabetes. Type 2 diabetes happens when the body can't respond normally to insulin. Symptoms are similar to those of type 1 diabetes. Many children and teens who develop type 2 diabetes are overweight, and this is thought to play a role in their decreased responsiveness to insulin. Some teens can be treated successfully with dietary changes, exercise, and oral medicine; others will need insulin injections. Controlling blood sugar levels reduces the risk of developing the same kinds of long-term health problems that happen with type 1 diabetes.
A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells.  Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD + into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.  Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD + /NADH form is more important in catabolic reactions, while NADP + /NADPH is used in anabolic reactions.