Faculty of Food Technology and Biotechnology, University of Zagreb
Abstract
Lipids are released in milk as fat globules, which are droplets of apolar lipids surrounded by a complex membrane deriving from the mammary epithelial cell (MEC) and called the milk fat globule membrane (MFGM). The structure of the MFGM is highly complex and closely related to the mechanisms of milk fat globule secretion in the mammary epithelial cell. Indeed, MFGM is composed of two biological membranes, a phospholipid monolayer, deriving from the endoplasmic reticulum, and a phospholipid bilayer, which originates from the apical plasma membrane of the MEC, with variable amounts of cytoplasm trapped between. Biochemical techniques (i.e. sodium dodecyl sulphate-polyacrylamide gel electrophoresis followed by different staining procedures) have been employed historically to characterize major MFGM proteins, namely MUC-1, fatty acid synthase, xanthine oxidase, butyrophilin, lactadherin, and adipophilin. However, recent advances in the field of proteomics (mostly development of one-dimensional gel electrophoresis approach coupled with tandem mass spectrometry) have led to the identification of hundreds of proteins associated with the MFGM. Surprisingly, newly identified MFGM proteins were not only involved in lipid metabolic or exocytosis-related biological processes, but also in cell signalling, translation, or host defense-related mechanisms. Therefore, the milk fat globule should no longer be viewed as an inert structure only devoted to the delivery of lipids to the newborn, but rather as a dynamic and informative compartment which can contribute to the improvement of our comprehension of the mammary gland biology.Lipidi se oslobađaju u mlijeku kao masne globule, tj. kapljice nepolarnih lipida okružene složenom membranom, koja se sastoji od epitelnih stanica mliječnih žlijezda, a nazivamo ju membrana masne globule. Struktura je te membrane iznimno složena, a utječe na mehanizam izlučivanja globule u epitelnu stanicu mliječne žlijezde. Sastavljena je od dviju bioloških membrana, fosfolipidnog sloja (koji potječe od endoplazmatskog retikuluma) i fosfolipidnog dvosloja (od plazmatske membrane epitelnih stanica vrha mliječne žlijezde), a između ta dva sloja nalazi se citoplazma različite debljine. Biokemijske su tehnike, poput SDS-PAGE s različitim postupcima bojanja, desetljećima primjenjivane za karakterizaciju glavnih proteina membrane masne globule, kao što su: MUC-1, sintaza masnih kiselina, ksantin oksidaza, BTN, MFG-E8 i ADPH. Novija dostignuća na polju protemike (većinom vezana uz razvoj 1D-LC-MS/MS metode) omogućila su identifikaciju stotinu proteina povezanih s membranom masne globule. Ti proteini sudjeluju u metabolizmu lipida i procesima egzocitoze, te u prenošenju signala, translaciji proteina i obrambenim mehanizmima domaćina. Stoga masnu globulu ne možemo više promatrati samo kao inertnu strukturu koja mastima opskrbljuje dojenčad već i kao dinamičan i informativan dio stanice, pomoću kojega se bolje može razumjeti biologija mliječnih žlijezda
