Summary
The LDH-isoenzymes of human blood platelets show a distinct predominance of the isoenzymes
2 and 3 upon chromatography on DEAE-cellulose. Small amounts of LDH-1 are also present,
while only traces of LDH-4 and -5 can be detected.
Enzyme kinetic investigations of the principal isoenzymes LDH-1, -2 and -3 clearly
show that the differences in inhibition constants with pyruvate as substrate which
are demonstrable at 25° largely disappear at 37°. On the other hand, the differences
among the isoenzymes in their affinity for pyruvate and lactate as substrate as well
as in with respect to the optimal substrate concentrations of pyruvate are more marked
at 37° than at 25°. Also, the type of inhibition found with lactate as substrate is
increasingly the expression of a higher order reaction in going from LDH-1 to LDH-3.
A dependence of the LDH distribution pattern upon the metabolism of the cell is discussed.
A comparison of our results with thrombocytes with those of other workers with erythrocytes
and leucocytes makes it unlikely that the LDH pattern is directly dependent upon the
existence of an oxidative metabolism. Rather, the redox potential of the cell could
be of importance for the nature of the pattern of isoenzymes and for their differing
kinetic properties.
