Phycobilisomes of Porphyridium cruentum. I. Isolation

A procedure was developed for the isolation of phycobilisomes from Porphyridium cruentum. The cell homogenate, suspended in phosphate buffer (pH 6.8), was treated with 1% Triton X-100, and its supernatant fraction was centrifuged on a sucrose step gradient. Phycobilisomes were recovered in the 1 M sucrose band. The phycobilisome fraction was identified by the characteristic appearance of the phycobilisomes, and the absorbance of the component pigments: phycoerythrin, R-phycocyanin, and allophycocyanin Isolated phycobilisomes had a prolate shape, with one particle axis longer than the other. Their size varied somewhat with their integrity, but was about 400-500 A (long axis) by 300-320 A (short axis). Phycobilisome recovery was determined at six phosphate buffer concentrations from 0.067 M to 1.0 M. In 0.5 M phosphate, phycobilisome yield (60%) and preservation were optimal. Such a preparation had a phycoerythrin 545 nm/phycocyanin 620 nm ratio of 8.4. Of the detergents tested (Triton X-100, Tween 80, and sodium deoxycholate), Triton X-100 gave the best results Freezing of the cells caused destruction of phycobilisomes.