Abstract
A substantial portion of the primary structure of pig liver glyceraldehyde-3-phosphate dehydrogenase has been investigated and the results compared with those previously reported for the pig muscle enzyme. Liver and muscle glyceraldehyde-3-phosphate dehydrogenases show the same amino acid content, and the first N-terminal 13 residues occur in the same sequence. No differences in N-terminal residues and amino acid composition have been evidenced by analysis of several tryptic peptides, which account for about 50% of the total amino acid sequence. From the electrophoretic mobilities of peptides T 8 T 9 and T 25 it is concluded that residues Asp 60, Asp 67 and Glu 220 in the reported sequence of the pig muscle enzyme must be present as amides in the liver enzyme. The NAD + content was found to be 2 mol per tetramer, while higher values have been reported for the muscle enzyme from various mammalian sources. The reactivity of lysyl side chains towards pyridoxal 5′-phosphate has been examined: the results indicate that Lys 212 is the main site reacted in fully inactivated pig liver holoenzyme. A similar result has been found for rabbit muscle apoenzyme, whereas rabbit muscle holoenzyme reacts at Lys 212 and 191.
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