Purified plasma membranes were obtained from five transplantable human
tumors, a
grade IV astrocytoma, an
oat cell carcinoma, and three
melanomas. Plasma membrane fractions were isolated from
tumor homogenates by differential and discontinuous
sucrose gradient centrifugation. Determination of
enzyme activities indicated that the plasma membranes were enriched 10- to 20-fold with respect to
5'-nucleotidase,
nicotinamide adenine dinucleotide glycohydrolase, Mg2+-activated
nucleoside triphosphatase, and
sialic acid. Specific activities of nearly all the
enzymes varied with the individual
tumors, even among
tumors of the same type, i.e., the
melanomas. Electron micrographs of the plasma membrane fractions showed smooth single-membrane vesicles with slight contamination by lysosomes. Therefore, these membranes are suitable for comparative biochemical studies and for the preparation of
tumor-specific
monoclonal antibodies. Plasma membranes from all five
tumors contained very high Mg2+-
adenosine triphosphatase (
ATPase) activities. The Na+-K+-
ATPase was a minor component of the total
ATPase of these membranes (less than 30%). The major component was an
ATPase exhibiting similar activity toward several
nucleoside triphosphates. The activity of such a
nucleoside triphosphatase has been correlated with tumorigenicity in cultured liver epithelial cells. The
nucleoside triphosphatase of the plasma membranes of
astrocytoma and
oat cell carcinoma was stimulated from 50 to 1005 by
concanavalin A, whereas
ATPase of the
melanoma plasma membranes was not or only slightly stimulated. The different response to
concanavalin A could be due to differences in the
ATPase molecules of the individual
tumors or to the different environment of the
ATPase.