Mode of uptake of sterol by Arthrobacter simplex

Abstract

The mechanism of uptake of water-insoluble β-sitosterol by a newly isolated strain of Arthrobacter simplex SS-7 was studied. The production of an extracellular sterol-pseudosolubilizing protein during growth of A. simplex on β-sitosterol was demonstrated by isolating the factor from the cell-free supernatant and its subsequent purification by Sephadex G-150 column chromatography. The M _r of the purified sterol-pseudosolubilizing protein determined by SDS–PAGE was 19 kDa. The rate of sterol pseudosolubilization (5.2 × 10⁻³ g l⁻¹ h⁻¹) could not adequately account for the rate of sterol uptake (72 × 10⁻³ g l⁻¹ h⁻¹) and the specific growth rate (56 × 10⁻³  h⁻¹). However in the unfavourable growth condition, when the cells were treated with sodium azide at the level of 30–60% of MIC, the sterol pseudosolubilization accounted for nearly 74% of the total growth containing 96% free cells. Cellular adherence to substrate particles was found to play an active role in the normal growth of the strain on β-sitosterol. Unlike sodium acetate-grown cells, whose surface activity was negligible (60 mN m⁻¹), the sterol-grown cells had strong surface activity (40 mN m⁻¹). The high lipid content and long chain fatty acids in the cell-wall of β-sitosterol-grown cells probably contribute to the high sterol adherence activity of the cells.