Growth of a yeast mutant on ring a modified cholesterol derivatives

Role of Cholesterol and its Biosynthetic Precursors on Membrane Organization and Dynamics: A Fluorescence Approach

Cholesterol is the most representative sterol present in membranes of higher eukaryotes, and is the end product of a long and multistep biosynthetic pathway. Lathosterol and zymosterol are biosynthetic precursors of cholesterol in Kandutsch-Russell and Bloch pathways, respectively. Lathosterol differs with cholesterol merely in the position of the double bond in the sterol ring, whereas zymosterol differs with cholesterol in position and number of double bonds. In this work, we have monitored the effect of cholesterol and its biosynthetic precursors (lathosterol and zymosterol) on membrane organization and dynamics in fluid and gel phase membranes. Toward this goal, we have utilized two fluorescent membrane probes, DPH and its cationic derivative TMA-DPH. Our results using these probes show that cholesterol and its biosynthetic precursors (lathosterol and zymosterol) exhibit similar trend in maintaining membrane organization and dynamics (as reported by fluorescence anisotropy and apparent rotational correlation time), in fluid phase POPC membranes. Notably, although lathosterol and zymosterol show similar trend in maintaining membrane organization and dynamics, the corresponding change for cholesterol is different in gel phase DPPC membranes. These results demonstrate that the position and number of double bonds in sterols is an important determinant in maintaining membrane physical properties. Our results assume significance since accumulation of precursors of cholesterol have been reported to be associated with severe pathological conditions.

Differential effect of cholesterol and its biosynthetic precursors on membrane dipole potential

Dipole potential is the potential difference within the membrane bilayer, which originates due to the nonrandom arrangement of lipid dipoles and water molecules at the membrane interface. Cholesterol, a representative sterol in higher eukaryotic membranes, is known to increase membrane dipole potential. In this work, we explored the effects of immediate (7-DHC and desmosterol) and evolutionary (ergosterol) precursors of cholesterol on membrane dipole potential, monitored by the dual wavelength ratiometric approach utilizing the probe di-8-ANEPPS. Our results show that the effect of these precursors on membrane dipole potential is very different from that observed with cholesterol, although the structural differences among them are subtle. These results assume relevance, since accumulation of cholesterol precursors due to defective cholesterol biosynthesis has been reported to result in several inherited metabolic disorders such as the Smith-Lemli-Opitz syndrome. Interestingly, cholesterol (and its precursors) has a negligible effect on dipole potential in polyunsaturated membranes. We interpret these results in terms of noncanonical orientation of cholesterol in these membranes. Our results constitute the first report on the effect of biosynthetic and evolutionary precursors of cholesterol on dipole potential, and imply that a subtle change in sterol structure can significantly alter the dipolar field at the membrane interface.

Inhibition of sterol biosynthesis by ergosterol and cholesterol in Saccharomyces cerevisiae

When accumulation of squalene was used as a measure of the flow of carbon into the sterol pathway in whole cells of semi-anaerobic Saccharomyces cerevisiae, both ergosterol and cholesterol were found to be inhibitory. However, at equivalent concentrations in the medium ergosterol was substantially the more potent inhibitor. Marked differences found in the absorption and esterification of the two sterols failed to account for the observed difference in their capacities to act as feedback agents. Cholesterol was much more effectively absorbed as well as esterified, but, when the abilities of the two sterols to lower the squalene level were calculated on the basis of free sterol in the cells, ergosterol remained more effective by a factor of four.

Stereochemically distinct roles for sterol in Saccharomyces cerevisiae

Cholesterol, (E)- but not (Z)-17(20)-dehydrocholesterol, 5 alpha-cholestan-3 beta-ol, sitosterol, and certain other sterols lacking a 24 beta-methyl group will replace (spare) most but not all of the 24 beta-methylsterol which has recently been found to be absolutely necessary for growth of oxygen-deprived wild type Saccharomyces cerevisiae in the presence of 2,3-iminosqualene. The results imply the existence of two stereochemically distinct roles for sterol in this organism. One of them (perhaps regulatory) requires, whereas the other (probably playing the so-called "bulk" membranous role) does not require the presence of the 24 beta-methyl group. The latter function, for which most of the sterol is needed, can be performed by various 24-alkyl- and 24-desalkylsterols.