Tyrosine hydroxylase (TH) demonstrates by two-dimen-sional electrophoresis amicroheterogeneitybothas a soluble recombinant humanTH(hTH1) andas amembrane-bound bovine TH (bTHmem). Part of the heterogeneity is likely due to deamidation of labile asparagine residues. Wild-type (wt)-hTH1was foundtobeasubstrate for theubiquitin(Ub) conjugating enzyme systemina reconstitutedin vitrosystem.