Structural studies on meiosis-activating sterols and structurally related compounds : potential ligands of the FF-MAS receptor

Abstract

Meiosis Activating Sterols (MAS) are key regulatory factors in the meiotic cell cycle. Two compounds in this family, Follicular Fluid-MAS (FF-MAS or 4,4-dimethyl-5a-cholest-8,14,24-triene-3b-ol) and Testicular-MAS (T-MAS or 4,4-dimethyl-5a-cholest-8,14-diene-3b-ol), have been isolated and identified. FF-MAS occurs in vivo in the synthetic pathway of cholesterol and was found to initiate the resumption of the arrested development of immature egg cells (i.e. primary oocytes), whereas T-MAS regulates the continuous generation of sperm cells. The biological function of these molecules makes them promising leads in the development of novel contraceptive agents, as well as drugs that remedy fertility problems that are related to the processes in which the MAS compounds are involved. The work described in this thesis explores the structural and physicochemical properties of MAS, in particular of FF-MAS and derived compounds, and the implications of these properties on in vivo activity. A stability analysis of various double and single bond isomers of MAS compounds, based on heat of formation calculations, is described. In addition, the structural and conformational analysis of the MAS and related molecules by crystallographic, NMR and computational methods is reported. A comparison is made between MAS compounds and vitamin D3 analogues with respect to the flexible aliphatic chain that both parent compounds possess. To this end, a homology model of the vitamin D3 receptor (VDR) is constructed and subsequent docking studies of its ligands are performed. Finally, an implementation of the program SUPERSTAR based on data from the Brookhaven Protein Databank is presented and applied to the homology model of the VDR mentioned above. The combined analysis of all data allows us to propose a pharmacophore model of the MAS compounds, which incorporates the conformation of the flexible aliphatic side chains and electrostatic properties and over-all shape requirements of the rigid fused-ring core structure.