323-331Feeding of protein deficient diet is known
to alter the transmembrane signalling in brain of rat by reducing total protein
kinase C (PKC) activity. Phospholipid metabolism regulates the activation of
PKC through generation of second messengers and the extent of PKC activation accordingly
influences the magnitude of phosphorylation of its endogenous substrate
proteins.Thus it was speculated that ingestion
of protein deficient diet may modify the turnover rate of membrane phospholipids
and magnitude of phosphorylation of endogenous substrate proteins of PKC. The
experiments were conducted on rats fed on three different types of laboratory prepared
diets viz. casein (20% casein), deficient (4% protein, rice flour as source of
protein ) and supplemented (deficient diet supplemented with L-lysine and DL-threonine)
for 28 days. The metabolism of phosphoinositides (PIs) and phosphatidyl choline
(PC) was studied by equilibrium labeling with [3H] myo inositol
and [14C methyl] choline chloride respectively. The phosphorylation
of endogenous substrate proteins of PKC was studied by using 32P-γ-ATP
followed by SDS-PAGE and autoradiography. The results suggest that in deficient
group, there is an increased incorporation of [3H] myo inositol
in PIs and inositol phosphate pool in comparison to the casein group. The phosphatidyl
inositol (PI) turnover reduced, although there was a marginal increase in the
phosphatidyl inositol monophosphate (PIP) and phosphatidyl inositol bis phosphate
(PIP2). Supplementation of diet showed a reversal of the pattern towards
control to a considerable extent. In the deficient group, PC metabolism showed
an increased incorporation of [14C methyl] choline in choline
phospholipids but decreased incorporation in phosphoryl choline in comaprison
with the casein group. The increase in total PC contents was significant but marginal
in residue contents. The turnover rate of PC increased only marginally and that
of residue declined. Supplementation of diet reduced the total contents of PC
and residue, but the turnover rate of PC and residue remained still higher.
Phosphorylation of endogenous proteins showed four different proteins of 78,
46, 33 and 16 kDa to be the substrates of PKC in casein group. In deficient group,
phosphorylation of these proteins increased markedly while supplementation of diet
had a reversing effect rendering the values to be intermediate between casein and
the supplemented group. The changes in phospholipid metabolism and in phosphorylation
of endogenous substrate proteins of PKC suggest that dietary protein deficiency
causes alterations in transmembrane signalling mechanism in rat brain. These effects
are partially reversed by improving the quality of proteins in the diet