Formation, Chromophore Composition, and Labeling Specificity of Cyanidium caldarium Phycocyanin

Phycocyanin is an accessory pigment found in the photosynthetic apparatus of blue green, cryptomonad, and red algae (1). The phycocyanin molecule consists of a protein to which are bound approximately 20 to 30 residues of the straight-chained tetrapyrrolic (bile pigment) chromophore, phycocyanobilin (10). Conventional methods used to determine the chromophore composition of proteins with polypyrrolic prosthetic groups, e.g., hemoglobin, myoglobin ,(5), which rely on quantitative splitting and recovery of the pigment constituents, fail to liberate phycocyanobilin from phycocyanin apoprotein (Troxler, unpublished observations). Phycocyanobilin is released from trichloroacetic aciddenatured phycocyanin, however, during reflux in absolute methanol albeit in limited yield as first described by Siegelman et al. i(,13,14). Cells of the alga, Cyanidium caldarium, synthesize phycocyanobilin-labeled phycocyanin-14C (16, 17) when illuminated in liquid suspension containing the porphyrin-bile pigment precursor, A-aminolevulinic acid-4-14C (ALA). The percent phycocyanobilin (by weight) in phycocyanin can be determined from the ratio of the specific radioactivity (dpm/,jg) of the chromophore to that of the entire bile pigment-protein complex. The specific radioactivity ratios method is valid, however,. only if labeling in the molecule is restricted to the prosthetic group. The present communication describes the quantitative relationship between the chromophore and apoprotein in C. caldariumt phycocyanin, and the specificity of ALA labeling in the phvcocyanin molecule.