Examination of prostasomes, isolated from human seminal plasma, showed
that there was very little remaining paranitrophenylphosphatase activ
ity when assayed in the presence of 10 mmol/l of tartrate and 2 mmol/l
of levamisole. Under these conditions it was possible to study the pr
ostasome membrane-bound 5'-nucleotidase activity, which was unaffected
by these two inhibitors. The activity was considered to be located at
the external surface of the prostasome membrane and a 50-60% increase
in activity was obtained by the addition of 0.05% Triton X-100. The p
rostasome membrane-linked 5'-nucleotidase readily hydrolysed 5'-AMP. T
wo other 5'-nucleoside monophosphates, 5'-IMP and 5'-GMP, were also hy
drolysed, but more slowly; 2'- or 3'-AMP were practically not attacked
. The prostasome membrane-linked 5'-nucleotidase obeyed Michaelis-Ment
en kinetics. Apparent K(m) for 5'-AMP was 11.2 +/- 2.1 mumol/l and Vna
, 64.7 +/- 11.4 nmol/mg protein/min. These figures were somewhat chang
ed in presence of 0.05% Triton X-100, the K(m) value being reduced by
30% and the V(max) value increased by 60%. Adenosine 5' (alpha, beta m
ethylene) diphosphate (100 mumol/1), Ni2+ (10 mmol/1) and concanavalin
A (20 mug/ml) were all potent inhibitors of the prostasome membrane-l
inked 5'-nucleotidase.