Wolf R A
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
Am J Physiol. 1990 Dec;259(6 Pt 1):C987-94. doi: 10.1152/ajpcell.1990.259.6.C987.
Compartmentation of phosphoinositide synthesis and transfer of endogenous phosphatidylinositol (PI) were characterized in membrane fractions prepared from rabbit myocardium. De novo synthesis of PI was highly enriched in free sarcoplasmic reticulum (551 pmol.mg-1. min-1) compared with that in sarcolemma (26.8 pmol.mg-1. min-1) and junctional sarcoplasmic reticulum (178 pmol.mg-1. min-1). In contrast, PI phosphorylation was highly enriched in sarcolemma (2.69 nmol.mg-1.min-1) compared with that in free sarcoplasmic reticulum (0.22 nmol.mg-1.min-1) and junctional sarcoplasmic reticulum (0.38 nmol.mg-1.min-1). Phosphorylation of endogenous phosphatidylinositol 4-phosphate to phosphatidylinositol 4,5-bisphosphate was also enriched in sarcolemma (38.5 pmol.mg-1.min-1) compared with that in free sarcoplasmic reticulum (less than 5.0 pmol.mg-1.min-1) and junctional sarcoplasmic reticulum (6.5 pmol.mg-1.min-1). Transfer of endogenous PI was characterized as a mechanism to overcome compartmentation of PI synthesis in cardiac membranes. A 29-kDa PI transfer protein was purified 1,500-fold from cytosol of rabbit myocardium. Both cytosol and purified PI transfer protein catalyzed the transfer of endogenous PI from microsomal sites of synthesis to sarcolemma. In conclusion, synthesis of PI is highly enriched in free sarcoplasmic reticulum, whereas phosphorylation of phosphoinositides is highly enriched in sarcolemma. A 29-kDa PI transfer protein in myocardial cytosol can mediate in vitro transfer of de novo-synthesized PI to the sarcolemma.
对从兔心肌制备的膜组分中磷酸肌醇合成的区室化以及内源性磷脂酰肌醇（PI）的转移进行了表征。与肌膜（26.8 pmol·mg⁻¹·min⁻¹）和连接肌质网（178 pmol·mg⁻¹·min⁻¹）相比，游离肌质网中PI的从头合成高度富集（551 pmol·mg⁻¹·min⁻¹）。相反，与游离肌质网（0.22 nmol·mg⁻¹·min⁻¹）和连接肌质网（0.38 nmol·mg⁻¹·min⁻¹）相比，肌膜中PI磷酸化高度富集（2.69 nmol·mg⁻¹·min⁻¹）。内源性磷脂酰肌醇4-磷酸磷酸化为磷脂酰肌醇4,5-二磷酸在肌膜中也高度富集（38.5 pmol·mg⁻¹·min⁻¹），而游离肌质网中该过程低于5.0 pmol·mg⁻¹·min⁻¹，连接肌质网中为6.5 pmol·mg⁻¹·min⁻¹。内源性PI的转移被表征为一种克服心肌膜中PI合成区室化的机制。从兔心肌胞质溶胶中纯化出了1500倍的29 kDa PI转移蛋白。胞质溶胶和纯化的PI转移蛋白均催化内源性PI从微粒体合成位点转移至肌膜。总之，PI的合成在游离肌质网中高度富集，而磷酸肌醇的磷酸化在肌膜中高度富集。心肌胞质溶胶中的29 kDa PI转移蛋白可在体外介导从头合成的PI向肌膜的转移。
