GLOSSARY

Cholesterol 7 Alpha-Hydroxylase (CYP7A1) catalyzes a major step in classical bile-acid synthesis from cholesterol.

The liver receives nutrients from the intestine and systemic circulation and can transform, store or release them. First pass generally concerns removal before systemic appearance and, depending on the definition, includes intestinal and hepatic contributions. Net hepatic extraction does not reveal every intrahepatic fate; concentration, blood flow and the measurement boundary must be specified before calculating it.

Fatty-acid synthesis builds chains from smaller units; lipogenesis can more broadly include lipid production and storage. De novo synthesis differs from esterifying existing fatty acids. Liver and adipose contributions depend on nutritional state and require energy and reducing equivalents. Eating carbohydrate does not immediately turn all of it into fat. Stores, oxidation, total intake and time determine carbon routing, so one synthetic pathway should not be treated as the inevitable fate of every carbohydrate meal.

For example, after training changes an enzyme result, first ask whether it was measured in blood, tissue or purified protein. Then assess assay conditions, nutrition and time since the last session alongside actual performance; this comparison helps distinguish a regulatory response from a nutritional deficiency or a presumed supplement need.

Location determines access to substrates, cofactors and local conditions. Cytosol, mitochondrial matrix, peroxisomes and nucleus are not equivalent environments. Microsomal usually refers to a laboratory membrane fraction largely derived from endoplasmic reticulum, not a normal independent organelle in living cells. Direct transfer of intermediates between enzymes requires evidence; proximity or membership of the same compartment alone does not establish substrate channeling or its magnitude.

A cofactor is a nonprotein component needed by some enzymes and may be an inorganic ion or an organic molecule. A coenzyme is organic, while a prosthetic group is generally tightly associated. Apoenzyme denotes protein lacking a required cofactor, and holoenzyme the complete active form. Binding strength and role must be specified: not every substance near an enzyme is its essential cofactor.

Flux control is usually distributed across steps, substrate supply and demand. Rate-limiting is condition-dependent rather than identifying an enzyme that always commands the whole pathway. A committed step marks more specific entry into a pathway and need not carry the greatest control. Increasing one enzyme or supplying its precursor does not guarantee greater network flux or performance without considering the wider system.

Measured activity is conversion per time under assay conditions rather than simply protein abundance. A substrate-rich assay may reveal potential capacity, while living tissue has different constraints and regulation. Units, tissue normalization and preparation methods are needed for comparison. Blood activity is not identical to intramuscular activity; laboratory changes require exercise timing, symptoms and tissue origin rather than being read as direct performance measures or a standalone diagnosis.