There are many signals that control catabolism. Most of the known signals are hormones and the molecules involved in metabolism itself. Endocrinologists have traditionally classified many of the hormones as anabolic or catabolic, depending on which part of metabolism they stimulate. The so-called classic catabolic hormones known since the early 20th century are cortisol , glucagon , and adrenaline (and other catecholamines ). In recent decades, many more hormones with at least some catabolic effects have been discovered, including cytokines , orexin (also known as hypocretin ), and melatonin . [ citation needed ]
For example, when the cell needs to produce specific proteins, it produces only enough of each of the various amino acids needed to synthesize those proteins. Moreover, certain amino acids are used by the cell to make glucose, which appears in the blood , or glycogen, a carbohydrate stored in the liver. So the products of amino acid catabolism do not accumulate, but rather feed the anabolic pathways of carbohydrate synthesis. Thus, while many organisms store energy-rich nutrients such as carbohydrates and fat , most do not store other biomolecules, such as proteins, or nucleic acids, the building blocks of deoxyribonucleic acid (DNA) .