The conversion of phytoene into fl-carotene was demonstrated previously in chromoplast membranes prepared from Capsicum fruits (B. Camara et al. 1982 Eur J Biochem 127: 255-258). The direct cyclization of lycopene into fl-carotene and the successful solubilization of the enzymic activity involved in this reaction is reported.incubation time, the reaction was stopped by the addition of 4 ml of chloroform:methanol (2:1, v/v). Carrier amounts (100 ,g) of i-carotene, y-carotene, a-carotene, and lycopene were added to facilitate the location of the reaction products. Preliminary chromatography was achieved on Silica Gel G plates developed with 40 to 60°C petroleum ether ether (99:1, v/v), which separates the bicyclic carotene fraction (a-carotene and ,Bcarotene with a higher RF value) from lycopene (lower RF value). The Through the use of isolated chromoplast membranes, we have shown that labeled phytoene is desaturated, isomerized, and cyclized to yield ,B-carotene (6). Lycopene plays a mandatory role in this sequence of reactions in that it is the direct substrate for cyclization, as shown by Camara and Moneger (7). We report here the direct cyclization oflycopene by Capsicum chromoplast membranes and the solubilization of the enzymic activity involved.MATERlALS AND METHODS Capsicum Chromoplast Membranes. The chromoplast membranes were prepared as described previously (5). The medium used for osmotic shock contained: 1 mM MgCl2, 10 mm DTT, and 50 mm Tris-HCl buffer (pH 7.6). The resulting homogenate was frozen overnight at -20°C before centrifugation at 100,000g to obtain a pellet containing membranes.Solubilization of Lycopene Cyclase Activity. The above membrane fraction was dispersed in 20 ml of buffer solution containing 1 mM MgC92, 10 mm DTT, and 50 mm and quickly added to 200 ml of freshly distilled acetone kept at -20C. The aggregates were dispersed with a Teflon pestle. The solution was filtered within 1 min and the residue was extracted again using the same procedure. The residual acetone powder was washed twice with 50 ml of freshly distilled ether at -20°C. The dried powder was extracted with a buffer solution containing 5 mM DTT and 100 mm K2HPO4-KH2PO4 buffer (pH 7). After centrifugation at 100,000g for 4 h, the supernatant was used for enzymic determinations.