4-hydroxyphenylacetaldehyde dehydrogenase  In enzymology, a 4-hydroxyphenylacetaldehyde dehydrogenase () is an enzyme that catalyzes the chemical reaction  The 3 substrates of this enzyme are 4-hydroxyphenylacetaldehyde, NAD+, and H2O, whereas its 3 products are 4-hydroxyphenylacetate, NADH, and H+. This enzyme belongs to the family of oxidoreductases, specifically those acting on the aldehyde or oxo group of donor with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is 4-hydroxyphenylacetaldehyde:NAD+ oxidoreductase. This enzyme is also called 4-HPAL dehydrogenase. This enzyme participates in tyrosine metabolism.