Catalyzes the reversible NAD(+)-dependent oxidative deamination of L-phenylalanine to phenylpyruvate. H2O + L-phenylalanine + NAD(+) = 3-phenylpyruvate + H(+) + NADH + NH4(+) Thermostable. Amino-acid biosynthesis; L-phenylalanine biosynthesis; L-phenylalanine from phenylpyruvate (PDH route): step 1/1. Belongs to the Glu/Leu/Phe/Val dehydrogenases family.