The effects of pyrroloquinoline quinone on heme-regulated eIF-2α kinase and eIF-2B activities in eukaryotic protein synthesis

Abstract

Pyrroloquinoline quinone (PQQ), a novel cofactor of biological redox processes, is ubiquitous in animal cells. We have examined the effects of PQQ on protein synthesis. PQQ inhibits protein synthesis in hemin-supplemented rabbit reticulocyte lysates. This inhibition is characterized by increased phosphorylation of eIF-2α and by diminished guanine nucleotide exchange activity of eIF-2B. The increased eIF-2α phosphorylation is the result of activation by PQQ of the heme-regulated eIF-2α kinase (HRI). The addition of 10 μM PQQ completely inhibits the increase in protein synthesis that occurs on the addition of hemin (20 μM) to heme-deficient lysates, whereas a lower concentration of PQQ (100 nM) causes a very slight stimulation of protein synthesis. The increased eIF-2α phosphorylation that occurs at high concentrations of PQQ inhibits eIF-2B activity, presumably due to formation of a 15S complex [eIF-2(αP)·eIF-2B] in which eIF-2B becomes non- functional. Low concentrations of PQQ (0.1-1 μM) do not affect eIF-2α phosphorylation, but rather enhance the guanine nucleotide exchange activity of eIF-2B in reticulocyte lysates. In Chinese hamster ovary cell extract which is devoid of significant eIF-2α kinase activity, addition of both low and high concentrations of PQQ results in an increase in eIF-2B activity. The addition of PQQ to reticulocyte lysates activates HRI whereas addition of PQQ to purified HRI in vitro inhibits the autokinase and eIF-2α kinase activity of the HRI; the inhibition of purified HRI by PQQ is observed both in the presence and absence of hemin. These findings suggest that PQQ inhibits purified HRI by acting as an oxidant whereas in lysates in which PQQ is readily reduced, the PQQ acts as a reductant and increases the activities of both HRI and eIF-2B.