Neopterin and biopterin are by-products of the redox reactions involving tetrahydrobiopterin (BH4). BH4 functions as a cofactor for the enzymes responsible for the production of monoamine neurotransmitters (epinephrine, norepinephrine, DOPA, serotonin), and as a cofactor in nitric oxide production. Low biopterin levels may reflect insufficient BH4 status.