Characterization of bovine angiogenin-1 and lactogenin-like protein as glycyrrhizin-binding proteins and their in vitro phosphorylation by C-kinase

Angiogenin-1 (p15, an angiogenesis inducer with RNase activity) and lactogenin-like protein (p17) isolated front partially purified bovine lactoferrin (bLF) preparations were characterized as glycyrrhizin (GL)-binding proteins (gbPs), As expected, bLF-affinity column chromatography confirmed these two gbPs to be bLF-binding proteins. These two purified gbPs exhibited RNase activities when incubated with poly(C) as a substrate. Both CL and glycarrhetinic acid (GA) at 100 muM significantly inhibited RNase activities of these two gbPs, both of which functioned as phosphate acceptors of C-kinase in vitro, Phosphorylation of p15 and p17 by C-kinase was inhibited by GA in a dose-dependent manner with the 50% inhibition dose (ID50) of approx. 10 muM, whereas GL required a relatively high dose (300 muM) to inhibit significantly it. A GA derivative (oGA, ID50 = approx. 0.3 muM) was found to be a potent inhibitor of the C-kinase-mediated phosphorylation of these two gbPs in vitro. In addition, a possible physiological significance of C-kinase on the physiological interaction between bLF and two bLF-binding proteins (p15 and p17) is noted.