The inhibitory effects of
tetrahydropapaverine on
serotonin biosynthesis in
serotonin-producing murine
mastocytoma P815 cells were investigated.
Tetrahydropapaverine at concentration ranges of 5-20 microM decreased
serotonin content in a concentration-dependent manner in P815 cells and showed 42.1% inhibition of
serotonin content at 5.0 microM at 24 hr. The value of 50% inhibitory concentration, IC50, of
tetrahydropapaverine was 6.2 microM. Under these conditions,
tryptophan hydroxylase (EC 1.14.16.4, TPH) was inhibited for 24-36 hr
after treatment with
tetrahydropapaverine in P815 cells (49.1% inhibition at 7.5 microM). However,
aromatic L-amino acid decarboxylase activity was not affected by
tetrahydropapaverine. In addition,
tetrahydropapaverine inhibited the activity of TPH, prepared from the P815 cells (P815-TPH), with the IC50 value of 5.7 microM.
Tetrahydropapaverine un-competitively inhibited P815-TPH with the substrate
L-tryptophan, and non-competitively with the cofactor DL-6-methyl-5,6,7,8-tetrahydropteridin. The Ki value of
tetrahydropapaverine with
L-tryptophan was 10.1 microM. These data indicate that
tetrahydropapaverine leads to a decrease in
serotonin content by the inhibition of TPH activity in P815 cells.