J. Biol. Chem., Vol. 260, Issue 30, 16052-16055, Dec, 1985
5-Methyltryptamine stimulates phospholipase C-mediated breakdown of exogenous phosphoinositides by blowfly salivary gland membranes
I Litosch and JN Fain
5-Methyltryptamine, through a GTP-dependent mechanism, stimulated breakdown
of endogenous [3H]inositol-labeled phosphoinositides in membranes prepared
from blowfly salivary gland homogenates through a phospholipase C
exhibiting a pH optimum of approximately 7.0. Unlabeled membranes, prepared
from salivary gland homogenates, hydrolyzed exogenous
[3H]phosphatidylinositol 4,5-bisphosphate substrate with generation of
labeled inositol phosphates. Inositol trisphosphate formation was increased
approximately 200% by 10 microM guanosine 5'-(O- thio)-trisphosphate (GTP
gamma S) within 30 s. 5-Methyltryptamine, in the presence of 10 microM GTP
gamma S, increased the rate of inositol trisphosphate formation by
approximately 500% within 30 s. Half-maximal activation of
hormone-stimulated breakdown of exogenous substrate required approximately
0.05 microM GTP gamma S. [3H]Phosphatidylinositol was also hydrolyzed
during incubation with membranes, resulting in the generation of inositol,
glycerol phosphoinositol, and inositol monophosphate. Formation of inositol
monophosphate was stimulated approximately 30% by 10 microM GTP gamma S and
10 microM 5-methyltryptamine. Neither inositol nor glycerol phosphoinositol
formation was affected by hormone. These results indicate that in a
cell-free system from blowfly salivary glands, 5- methyltryptamine, through
a GTP-dependent mechanism, directly activates a phospholipase C which
mediates phosphoinositide hydrolysis.
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