Metabolism of lipids

Lipids are hydrophobic but otherwise chemically diverse molecules that play a wide variety of roles in human biology. They include ketone bodies, fatty acids, triacylglycerols, phospholipids and sphingolipids, eicosanoids, cholesterol, bile salts, steroid hormones, and fat-soluble vitamins. They function as a major source of energy (fatty acids, triacylglycerols, and ketone bodies), are major constituents of cell membranes (cholesterol and phospholipids), play a major role in their own digestion and uptake (bile salts), and participate in numerous signaling and regulatory processes (steroid hormones, eicosanoids, phosphatidylinositols, and sphingolipids) (Vance & Vance 2008 - URL).<p>The central steroid in human biology is cholesterol, obtained from animal fats consumed in the diet or synthesized de novo from acetyl-coenzyme A. (Vegetable fats contain various sterols but no cholesterol.) Cholesterol is an essential constituent of lipid bilayer membranes and is the starting point for the biosyntheses of bile acids and salts, steroid hormones, and vitamin D. Bile acids and salts are mostly synthesized in the liver. They are released into the intestine and function as detergents to solubilize dietary fats. Steroid hormones are mostly synthesized in the adrenal gland and gonads. They regulate energy metabolism and stress responses (glucocorticoids), salt balance (mineralocorticoids), and sexual development and function (androgens and estrogens). At the same time, chronically elevated cholesterol levels in the body are associated with the formation of atherosclerotic lesions and hence increased risk of heart attacks and strokes. The human body lacks a mechanism for degrading excess cholesterol, although an appreciable amount is lost daily in the form of bile salts and acids that escape recycling.<p>Aspects of lipid metabolism currently annotated in Reactome include lipid digestion, mobilization, and transport; fatty acid, triacylglycerol, and ketone body metabolism; peroxisomal lipid metabolism; phospholipid and sphingolipid metabolism; cholesterol biosynthesis; bile acid and bile salt metabolism; and steroid hormone biosynthesis.

脂类是疏水性但化学性质多样的分子，在人类生物学中扮演着多种角色。它们包括酮体、脂肪酸、三酰甘油、磷脂和鞘脂、二十烷类物质、胆固醇、胆汁盐、类固醇激素以及脂溶性维生素。脂类是主要的能量来源（脂肪酸、三酰甘油和酮体），是细胞膜的主要组成成分（胆固醇和磷脂），在自身的消化和吸收过程中发挥关键作用（胆汁盐），并参与众多信号传导和调控过程（类固醇激素、二十烷类物质、磷脂酰肌醇和鞘脂）。（Vance & Vance 2008 - URL）。人类生物学中的核心类固醇物质是胆固醇，它来源于饮食中摄入的动物脂肪或由乙酰辅酶A从头合成。植物脂肪含有各种类固醇，但不含胆固醇。胆固醇是脂质双层膜不可或缺的成分，是胆汁酸和盐、类固醇激素以及维生素D生物合成的基础。胆汁酸和盐主要在肝脏合成，进入肠道后作为洗涤剂溶解膳食脂肪。类固醇激素主要在肾上腺和性腺中合成，调控能量代谢和应激反应（糖皮质激素）、盐平衡（盐皮质激素）以及性发育和功能（雄激素和雌激素）。与此同时，体内胆固醇水平慢性升高与动脉粥样硬化病变的形成密切相关，从而增加了心脏病发作和中风的风险。人体缺乏降解过量胆固醇的机制，尽管每天有相当数量的胆固醇以胆汁盐和酸的形式丢失，逃避免疫循环。《Reactome》目前标注的脂质代谢方面包括脂质消化、动员和运输；脂肪酸、三酰甘油和酮体代谢；过氧化物酶体脂质代谢；磷脂和鞘脂代谢；胆固醇生物合成；胆汁酸和胆汁盐代谢；以及类固醇激素生物合成。