GLUTAMATE-DEHYDROGENASE FROM TUNA LIVER - PURIFICATION, CHARACTERISTICS AND SEQUENCE OF A PEPTIDE CONTAINING AN ESSENTIAL LYSINE RESIDUE

Glutamate dehydrogenase (EC 1.4.1.2-4) was purified and crystallized from the acetone powder of tuna liver. The enzyme has a MW of 333,000 .+-. 15,000 as evaluated by sedimentation equilibrium and consists of 6 identical subunits. Unlike the bovine enzyme the MW does not increase with increasing protein concentration, indicating that the tuna enzyme has no tendency to polymerize. The amino acid composition and peptide maps of the tuna and bovine liver enzyme are similar, suggesting considerable homology between the 2 enzymes. From tryptic digest, a hexadecapeptide containing a lysine residue reactive to pyridoxal 5''-phosphate exhibits the same composition and sequence as the peptide containing the reactive lysine-126 in the sequence of the bovine enzyme. The molecular activity is 25 and 510 mol of substrate/mol enzyme per s, respectively, for the glutamate oxidation and the .alpha.-ketoglutarate reduction with NAD or NADP as coenzymes. The enzyme is regulated by pyridine nucleotides like other vertebrate enzymes, but it also exhibits some coenzyme specificity. The activity is .apprx. 15 times higher with NAD than with NADP.