ENZYMES. — The name, which means “in yeast,” from the Greek εὐχῆ, was proposed by Kühn in 1878. Together with vitamins and hormones, enzymes constitute the so-called “biocatalysts,” substances that promote many physiological and biological processes of great importance for life.
The first enzymatic action was discovered by Spallanzani in 1720, when he noticed, while studying digestive phenomena, that gastric juice had the property of digesting meat. In 1830, Dubrunfaut found that the extract of germinated barley transformed starch into maltose. In 1836, Schwann gave the name *pepsinia* to the active principle contained in gastric juice capable of digesting meat, and a year later, Wöhler and Liebig called *emulsin* the active principle contained in preparations of bitter almonds that acted in the hydrolysis of amygdalin. Pasteur had discovered and demonstrated that fermentations are the work of microorganisms and that alcoholic fermentation was due to yeasts. But in 1867, Buchner succeeded in obtaining alcoholic fermentation by means of a substance extracted from yeast, a substance that was called *zymase* and was later recognized as consisting of various enzymes. Until the time when Buchner discovered zymase, enzymes were divided into *figurate* enzymes (or ferments) (consisting of living cells) and *non-figurate* enzymes (which acted, like pepsin, without the need for the presence of a living cell). But after Buchner’s discovery, a new classification was reached, into intracellular enzymes, which are contained within cells from which they can be extracted, and extracellular enzymes, which are spontaneously secreted by cells.
Ducaux proposed forming the name of the enzyme by adding the suffix *-ase* to the name of the compound on which the enzyme acts; for example, *amylase*, *lipase*, *protease*, etc., depending on whether the compound undergoing enzymatic action is starch, fat, protein, etc. The names *pepsinia*, *emulsin*, etc., remain as such because they had entered common usage before Ducaux’s proposal. In general, hydrolases are extracellular, while desmolases are intracellular. Hydrolases are enzymes that preside over hydrolysis phenomena similar to those produced by acids and bases; by the addition of one or more water molecules, they split the molecules on which they act into simpler ones. Desmolases, on the other hand, cause the breakdown of complex molecules into simpler ones through reactions different from those of hydrolysis.
Enzymes are further divided into several subgroups: carbohydrates (action on sugars or carbohydrates), proteases (action on proteins), lipases (action on fats), glucosidases (action on glucosides), dehydrogenases and oxidases (action of dehydrogenation and oxidation), and others.
The activity of enzymes is regulated by several factors: the nature of the substrate on which the enzyme acts, the concentration of the enzyme itself, temperature, and pH. Temperature is of great importance: within certain limits, an increase in temperature promotes enzymatic action. Almost all enzymes are irreversibly destroyed at 80°, meaning without the possibility of being reconstituted; a few enzymes are not destroyed at 80°: examples include crystalline ribonuclease and crystalline trypsin in hydrochloric acid N/10. The pH, that is, the concentration of hydrogen ions—or more simply, the degree of acidity or basicity of the environment in which the enzyme acts—is also of great importance. If, in fact, one exceeds or falls even slightly below the optimal pH, the enzyme loses its activity, first slowly and then more rapidly, until it becomes completely inactive, usually reversibly at first, and then definitively and irreversibly.
Generally, an enzyme consists of two parts: the coenzyme and the apoenzyme. The coenzyme is the stimulant of the apoenzyme; that is, it functions as an activator of the apoenzyme, which, without the former, would be incapable of any activity. The apoenzyme is generally a protein of a colloidal nature and, therefore, of high molecular weight. The coenzyme, on the other hand, consists of one or more different simple or complex substances. Even in small quantities, enzymes are capable of catalyzing the transformation of large quantities of substrate (see CATALYSIS).
Enzymes are found in all living cells. The most important are distributed as follows: 1) *Pepsinia*, which is present only in gastric juice, hydrolyzes proteins into peptones and, in some cases, even into amino acids. 2) *Trypsin*, present only in gastric juice after it has entered the intestine, that is, after the enterokinase of the intestinal mucosa has activated trypsinogen, transforming it into trypsin; unlike pepsinia, trypsin cannot attack native proteins but only after they have already been split into peptones and proteins. Hydrolysis proceeds until amino acids are formed. 3) *Lipase*, abundant in the pancreas but also present in the stomach, fatty tissues, and the small intestine; other examples are the phosphatases of serum, kidneys, intestinal epithelium, milk, yeast, etc. *Cholinesterase*, probably consisting of a mixture of two enzymes, one specific and one nonspecific, which are sometimes separate and sometimes united, found in the pancreas, liver, serum, and nervous tissues. The action of this enzyme is to hydrolyze acetylcholine; this is a process directly connected to the transmission of nerve impulses. 4) *Carbohydrases*, which regulate the fermentative processes of sugars: among these is lysozyme, which is present in nasal mucosa and has the property of destroying certain bacteria. 5) *Amylase*, which hydrolyzes starch and is abundant in the pancreas and salivary glands but is also present in the blood, muscles, and other tissues. Amylase and carbohydrases are also of great industrial importance, especially in the production of alcohol from starch.
Some enzymes have a vitamin-like function, and conversely, some vitamins have enzymatic action. This, for example, is the case with yellow enzyme, ribonuclease, ascorbic acid oxidase, and the conjugase of vitamin B.
Enzymes can be considered as the regulators of all biological functions of animal and plant organisms. It is truly interesting and wonderful to observe how these substances regulate and guide the outcome of biochemical reactions without appearing among the products of the reaction itself, and how the harmonious concatenation of numerous and successive enzymatic actions results in complex biological processes of great importance for life and industry: glycolysis or lactic fermentation carried out by muscle cells is a fine example of the sequence of enzymatic phenomena and their concrete orientation in the actual process of muscle contraction.