Catalyzes the reversible deamination of L-leucine to 4-methyl-2-oxopentanoate. H2O + L-leucine + NAD(+) = 4-methyl-2-oxopentanoate + H(+) + NADH + NH4(+) Amino-acid degradation; L-leucine degradation; 4-methyl-2-oxopentanoate from L-leucine (dehydrogenase route): step 1/1. Belongs to the Glu/Leu/Phe/Val dehydrogenases family.