Stimulation by chelerythrine of the phosphorylation of the amino acid serine in an approximately 20 kDa protein present in the mitochondrial fraction of the rat retina.

It has been reported that chelerythrine chloride, a benzophenanthridine alkaloid, with a wide variety of biologic effects stimulates the phosphorylation of an approximately 20 kDa protein present in the mitochondrial fraction of the rat retina. It has also been shown previously that both the serine and threonine resides in this phosphoprotein are phosphorylated when the retinal preparation is incubated in the presence of [gamma-32P]ATP. Phosphorylation of the serine residue(s) was determined to predominate over phosphorylation of the threonine residue(s). In the present investigation, it was demonstrated that chelerythrine stimulates the incorporation of radioactive phosphate into the serine residue(s), increasing the radioactivity in the phosphoserine/ phosphothreonine ratio by 80%. This observation represents a novel and apparently contradictory effect for chelerythrine, which is used normally as a selective protein kinase C inhibitor. In addition to testing chelerythrine for its effects on the phosphorylation of the approximately 20 kDa protein, a number of other protein kinase inhibitors and activators were investigated. The results suggest that the enzyme responsible for the phosphorylation of the approximately 20 kDa protein is not a well-characterized or documented kinase.