Quantitative degradation of radiolabeled phycobiliproteins. Chromic acid degradation of C-phycocyanin.

Abstract

Phycocyanin and allophycocyanin are bile-pigment apoprotein complexes found in the photo-synthetic apparatus of red and blue-green algae. Phycocyaninand allophycocyanin, in which the bile pigment (phycobilin) chromophore was radiolabeled (but the apoprotein was unlabeled), were prepared by incubating cells of the unicellular red alga, Cyanidum caldarium, with δ-amino[4-14C]-levulinic acid. The radiolabeled phycobiliproteins were isolated and purified, and the radiolabeled chromophore was cleaved from apoprotein by boiling in methanol. the chromophore-free acid (phycobilin1) was converted to the dimethylester (phycobilin 2) and the subunits of phycocyanin, allophycocyanin, and phycobilins 1–3 were subjected to chromic acid degradation at 25°C and 100°C. The imide fingerprints and quantiative recovery of imides from rings I–IV of phyco-bilins and phycobiliproteins (chromophore covalently attached) were determined. A different recovery pattern of imides from rings I–IV of phycobilins 1–3 was observed but the pattern was normally reproducible for a given phycobilin 1 after degradation at 100°C as was recovered at 25 °C. Chromic acid degradation of phycobiliproteins at 25 °C yielded rings II and IV as hematinic acid and mi=ethylethylmaleimide, respectively, in a 1 : 1 ratio. Chromic acid degradation at 100 °C yielded twice as much hematinic acid (rings II and III) and the same amount of methyl-ethylmaleimide as obtained at 25°C, in addition to ring I as 2-ehtylidene-3-methylsccinimide. These imide profiles and recovferies from phycobiliprotein. The imide fingerprints and recovery of imides from phycobilin 1 and phycobiliproteins are discussed in relation to the premise that rings I and III of the chromophore are covalently attached and to elucidation of phycobilin-apoprotein linkages by micro-degradative techniques.