As you may recall from chapter 5, the liver helps to regulate the blood glucose concentration by either removing glucose from the blood or adding glucose to it, according to the needs of the body. After a carbohydrate-rich meal, the liver can remove some glucose from the hepatic portal blood and convert it into glyco-gen and triglycerides through the processes of glycogenesis and compounds as benzopyrene (a carcinogen from tobacco smoke and charbroiled meat), polychlorinated biphenyls (PCBs), and dioxin. The enzymes are members of a class called the cytochrome P450 enzymes (not related to the cytochromes of cell respiration) that comprises a few dozen enzymes with varying specificities. Together, these enzymes can metabolize thousands of toxic compounds. Since people vary in their hepatic content of the different cytochrome P450 enzymes, one person's sensitivity to a drug may be greater than another's because of a relative deficiency in the appropriate cytochrome P450 enzyme needed to metabolize that drug.
The liver cells contain enzymes for the metabolism of steroid hormones and other endogenous molecules, as well as for the detoxication of such exogenous toxic lipogenesis, respectively. During fasting, the liver secretes glucose into the blood. This glucose can be derived from the breakdown of stored glycogen in a process called glycogenolysis, or it can be produced by the conversion of noncarbohydrate molecules (such as amino acids) into glucose in a process called gluconeogenesis. The liver also contains the enzymes required to convert free fatty acids into ketone bodies (ketogenesis), which are secreted into the blood in large amounts during fasting. These processes are controlled by hormones and are explained further in chapter 19.
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