1. A tetrapeptide (Trp-Met-Asp-Phe-NH2) used to test the secretion of digestive juice.
2. A pterin derivative that is a required cofactor for a number of enzymes; for example, in the conversion of l-phenylalanine to l-tyrosine; the inability to synthesize tetrahydrobiopterin is associated with forms of malignant hyperphenylalaninemia.
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