28
in the case of erythrocytes in man,
attempts to verify this
were unsuccessful.
A zinc-containing protein of human leucocytes has been
found which contains 0.3 per cent of zine per gram dry weight
and which can be differentiated clearly from carbonic anhydrase
(Vallee, Hoch and Hughes,
1954).
Vallee and Neurath (1955) have shown that carboxypeptidase
of bovine pancreatic juice is a zinc enzyme.
It is an exopepti-
dase which splits terminal amino acids from peptides having a
free alpha-carboxyl group adjacent to the peptide bond.
in the proportion of one atom per molecule of protein,
Zinc
is firmly
bound to the protein and constitutes a prosthetic group indispensable for enzyme activity.
Four dehydrogenases dependent upon diphosphopyridine
nucleotide
(DPN)
for their activity have been shown to contain
zinc that is essential for their action:
(1) yeast alcohol
dehydrogenase, YADH (Vallee and Hoch, 1955),
alcohol dehydrogenase,
(2) horse liver
LADH (Theorell, Nygaard and Bonnichsen,
1955 and Vallee and Hoch,
1956),
(3) glutamic dehydrogenase of
beef liver, LGDH (Vallee,Adelstein and Olsen, 1955) and (4)
lactic dehydrogenase of rabbit skeletal muscle, SLDH (Vallee,
Wacker,
Bartholomay and Robin,
1956).
The zinc in these enzymes
is firmly bound and essential to the enzyme activity,
as all are