Function of the factor I modules (FIMS) of human complement component C6.

In order to elucidate the function of complement component C6, truncated C6 molecules were expressed recombinantly. These were either deleted of the factor I modules (FIMs) (C6des-748-913) or both complement control protein (CCP) modules and FIMs (C6des-611-913). C6des-748-913 exhibited approximately 60-70% of the hemolytic activity of full-length C6 when assayed for Alternative Pathway activity, but when measured for the Classical Pathway, C6des-748-914 was only 4-6% as effective as C6. The activity difference between C6 and C6des-748-913 for the two complement pathways can be explained by a greater stability of newly formed metastable C5b* when produced by the Alternative Pathway compared with that made by the Classical Pathway. The half-lives of metastable C5b* and the decay of (125)I-C5b measured from cells used to activate the Alternative Pathway were found to be about 5-12-fold longer than those same parameters derived from cells that had activated the Classical Pathway. (125)I-C5 binds reversibly to C6 in an ionic strength-dependent fashion, but (125)I-C5 binds only weakly to C6des-FIMs and not at all to C6des-CCP/FIMs. Therefore, although the FIMs are not required absolutely for C6 activity, these modules promote interaction of C6 with C5 enabling a more efficient bimolecular coupling ultimately leading to the formation of the C5b-6 complex.     

Cholesterylene, a newly recognized tissue lipid, found at high levels in the cornea.

In the course of measuring the concentration of cholesterol in an opacified dog cornea by gas-chromatography, relatively large amounts of an unidentified non-saponifiable lipid were recognized. When the unknown lipid was subjected to gas chromatographic-mass spectral analysis it displayed a major ion at m/z 368 M+. and was identified as cholesta-3,5-diene, cholesterylene, by computer match with mass spectral-registry data. Cholesterylene was then shown to be present in the corneas of normal dogs, cows and humans, accounting for 20-25% of the total steroid-sterol in dog corneas and 5-10% in cow and human. Cholesterylene, which can be considered as an extremely nonpolar dehydration product of cholesterol, has not previously been recognized in animal tissues. Although the source of corneal cholesterylene is unknown, preliminary results suggest non-enzymatic formation from cholesterol.