Modulation of the effect of bilirubin on protein phosphorylation by lysine-containing peptides

Protein phosphorylation is an important mechanism for regulation of cell processes. Bilirubin inhibits phosphorylation of several peptides/proteins by a number of different kinases, and this may contribute to the toxic effects of bilirubin on cells, particularly neurons. Bilirubin binds to lysine residues on both albumin and ligandin. The ATP-binding subdomain II on protein kinases contains an invariant lysine, which might hypothetically be involved in mediation or modulation of bilirubin-inhibition of protein phosphorylation. We have studied the ability of lysine-containing peptides to modulate the effects of bilirubin, using phosphorylation of a phospholemman peptide catalyzed by protein kinase A as a model system. Addition of bilirubin (50 microM) decreased the activity of the catalytic subunit of protein kinase A by 75%. A synthetic lysine-containing decapeptide which mimicked part of subdomain II on the protein kinase family was partially able to prevent the bilirubin effects. Similar effects were not observed with two other decapeptides in which lysine had been replaced by arginine or alanine. Polylysine (100 microM) completely prevented the inhibitory effect of 50 microM bilirubin, whereas polyglutamate and polyarginine did not have this effect. Poly-D-lysine and poly-L-lysine appeared to be equivalent in their ability to prevent the bilirubin effect. These data support the notion that binding of bilirubin to lysine may play a role in the mediation and/or modulation of bilirubin neurotoxicity.