Calcium complexation by steroids involved in the steroidogenesis.

Steroids that take part in the pathways of human steroidogenesis are involved in many biological mechanisms where they interact with calcium. In the present work the binding selectivities and affinities for calcium of progestagens, mineralocorticoids, androstagens and estrogens were studied by ESI-MS. The adduct profile of each steroid was characterized by high resolution and tandem mass spectrometry. The relative stability of the most important adducts was studied by threshold collision induced dissociation, E1/2. Doubly-charged steroid-calcium complexes [nM+Ca]2+ with n=1-6 were predominant in the mass spectra. The adduct [5M+Ca]2+ was the base peak for most 3-keto-steroids, while ligands bearing hindered ketones or α-hydroxy-ketones also yielded [nM+Ca+mH2O]2+ with n=3-4 and m=0-1. Principal component analysis (PCA) allowed us to spot the main differences and similatities in the binding behavior of these steroids. The isomers testosterone and dehydroepiandrosterone (DHEA), androstanolone (DHT) and EPIA, and 17-α-hydroxyprogesterone (17P4) and 11-deoxycorticosterone (DOC) showed remarkable differences in their adduct profile. Computational modelling of representative adducts was performed by DFT methods. The possible binding modes at low and high number of steroid ligands were determined by calcium Gas Phase Affinity (CaGPA), and through modelling of the complexes and comparison of their relative stabilities, in agreement with the experimental results.