Lipozomes as a model system in biomineralization research

Sintetičke lipidne vezikule, lipozomi, su korišteni kao modelne membrane za izučavanje mineralizacije kalcij-fosfata. Kinetika nastajanja mineralne faze praćena je u vodenim suspenzijama anionskih lipozoma, koji su sadržavali visoku koncentraciju kolesterola i različite fosfolipide, te u modelnim lipozomima, čiji je sastav odgovarao lipidnom sastavu membrane vezikula matriksa. Rezultati istraživanja pokazuju da lipidni sastojci membrana utječu na taloženje kalcij-fosfata u lipozomnim suspenzijama na najmanje 3 načina: (1) kontroliranjem transporta kalcijevih iona kroz membrane, (2) usporavanjem prodiranja intralipozomalno nastalih kristala u medij izvan lipozoma i (3) inhibicijom ekstralipozomalne mineralizacije. Uz 50 mM enkapsuliranog topljivog anorganskog fosfata i u prisutnosti ionofore, kolesterol značajno utječe na kinetiku taložnog procesa. Inhibicija intralipozomalnog taloženja je u sustavima višeg sadržaja kolesterola rezultat djelovanja njegovih nefleksibilnih molekula, koje čine membranu znatno rigidnijom. Utjecaj kolesterola na transmembranske procese ne mijenja se ugradnjom različitih fosfolipida u lipozomalne membrane.Artificial lipid vesicles, liposomes, are used as a model for examining membrane-mediated calcium phosphate mineralization. Applying different procedures and techniques, the kinetics of mineral formation are followed in aqueous suspension of anionic liposomes containing high levels of cholesterol, different phospholipid — cholesterol compositions and model liposomes with matrix vesicle-like compositions. Results of the study show that membrane lipid constituents can affect calcium phosphate precipitation in liposome suspensions at least in 3 ways: (1) by controlling transmembrane transport of calcium ions, (2) by delaying the release of intraliposomally formed seed crystals into the external medium and (3) by inhibiting further growth of seed crystals once they come into contact with the external solution. With 50 mM encapsulated phosphate and in the presence of ionophore, cholesterol significantly affected the mineralization kinetics. Inhibition of ionophore-mediated intraliposomal mineral formation could be related to the inflexible cholesterol molecules making the liposomal membrane more rigid. Interference of cholesterol with the membrane transport processes necessary for endogenous precipitation in liposomes containing different anionic components is not compromised by the specificic phospholipid incorporated in the membrane