Hereditary fructose intolerance

Enzymes -- The Movers And Shakers Of Our Body Chemistry

Remember when you were a kid and your mother made junket rennet custard for you? If you do, you are remembering one of your first exposures to the use of enzymes outside the body. Rennet contains rennin, a naturally occuring enzyme that comes from cows. The enzyme partially digests milk and causes it to curdle.

Enzymes are protein-like substances found in plant and animal cells, including human cells. Enzymes play a number of important roles by acting as catalysts in starting or speeding up chemical reactions. In fact, the value of enzymes outside the body has led to their use not only in custard, but in other everyday products as well.

For example, papain--derived from papaya fruit -- is used in meat tenderizers to partially digest, or break down, meat protein, thus softening it. Enzymes are also used in laundry detergents to help dissolve grease and other difficult stains.

In order to understand the actions of enzymes, it is important to know that most of the chemicals in food are simply too large and complex for human body to use "as is." Proteins, for example, are long chains oa mino acids, and even common sugars and starches can be highly complex chemicals. Just as you cannot swallow large chunks whole, but must chew on bite-size pieces, most chemicals entering the body must be broken down before they can be put to use. Enzymes rearrange or split these chemicals into smaller "bite-size" pieces ready for further chemical reaction. For example, table sugar (sucrose) is really two simple sugars, chemically linked, that are separated by the enzyme sucrase during digestion. The resulting simple sugars, glucose and fructose, can then be used by the body.

Besides sucrase, other common digestive enzymes are amylase, pepsin, trypsin and lipase. Amylase is present in saliva and begins digestion of starches into simple sugars right in the mouth. That's why plain crackers will begin to taste a bit sweet after a few minutes of chewing.

Pepsin, found in gastric juice in the stomach, begins the job of splitting food proteins into smaller chemical units called "peptones." This task is continued in the small intestine with the addition of trypsin produced by the pancreas. Lipase, which is also produced by the pancreas, helps break up fat. Without these and other digestive enzymes, it would be difficult or impossible to absorb nutrients from many foods.

The human body does not produce enzymes to act on all substances. That's why humans can't digest cellulose. In fact, neither can termites, although they do eat wood, which is mostly cellulose. But in the case of termites, protozoa (one-celled animals) living inside the insect produce the necessary enzymes that split cellulose into digetible "pieces".

The duties of enzymes, however, go beyond digestion. An enzyme named renin (not to be cofused with the milk-curdling rennin) is involved in regulating blood pressure. Other enzymes are the foundation of our detoxification systems, helping to metabolize (break down) most "foreign" chemicals, such as drugs. There are entire systems of enzymes, each able to metabolize certain chemical groupings. For example, the liver produces an enzyme system capable of metabolizing barbiturate sedatives.

People who have impaired liver function may have difficulty eliminating some drugs. On the other hand, the barbiturates, as well as certain other drugs, stimulate production of enzymes that otherwise would not occur. These drugs are called "enzyme inducers," and the enzymes they induce may interfere with medical therapy by speeding the drughs own metabolism or that of other drugs a patient is taking at the same time. In fact, the longer an enzyme inducer is taken, the faster the body may eliminate it.

Knowledge of the way enzymes work has enabled advances in medical therapy. One such advance is Augmentin, an antibiotic recently approved by FDA. Augmentin is made of two chemicals, amoxicillin and clavulanate potassium, but only the first one is active against bacteria. The other is a compound specifically designed to block the kidney enzyme beta-lactamase from metabolizing the antibiotic portion, thus prolonging the drug's stay in the body and improving its effectiveness.

Often bacteria that are present in the body have their own enzyme systems that may complicate drug treatment. For example, some bacteria produce an enzyme called penicillinase that is capable of destroying penicillin. The mechanism can interfere with treatment of infections such as strep throat. To counter this problem, scientists have designed new members of the penicillin family that have some degree of resistance to penicillinase.

As with drugs, the body's processing of foods can also be hampered by certain enzyme deficiencies. One such deficiency that affects a fairly large proportion of the population is lactose intolerance. Lactose is a complex sugar found in milk. Milk is a highly complex food composed of fats, proteins, sugars and other nutrients. It requires considerable enzyme activity for proper digestion. But many people lack the enzyme lactase, which breaks lactose into simple sugars the body can use. When these lactase-lacking people consume products that contain milk, the result can be mild to severe indigestion. The deficiency is found less often among Cuacasian than non-Caucasian people. (See "Sweet Milk and Sour Stomachs" in the March 1984 FDA Consumer.)

Fortunately, lactase supplements that can be added to milk are available. Also, fermented milk products such as yogurt, aged cheeses and cottage cheese, in which some lactose is converted to other substances, may be better tolerated than non-fermented milk products.

Other enzyme deficiencies pose a more serious threat to health and well-being. One of these conditions, a deficiency in an enzyme known as glucose-6-phosphate dehydrogenase (G-6-P-D), was mentioned in a "M*A*S*H" television episode. In that segment, Corporal Maxwell Klinger became seriously ill when he was given a drug used to fight malaria. His sickness was due to a genetic absence of this enzyme--a deficiency common in persons of Mediterranean descent. Klinger's body simply could not metabolize the drug.

Klinger's particular problem is an example of a characteristic of many enzyme deficiencies. Generally, these deficiencies are distributed fairly evenly throughout the population. However, some deficiencies, such as G-6-P-D and lactose intolerance, may be concentrated in certain families, races, or even geographic areas due to revolutionary effects. G-6-P-D and lactose intolerance are both common in persons of Mediterranean descent.

Of the many such conditions, one of the earliest and best known--and most widely feared--is Tay-Sachs disease. This hereditary condition is mainly limited to persons of Eastern European Jewsish descent, although there is also noiceable incidence among Italian Catholics and a group of non-Jewish Canadians. Tay-Sachs disease is caused by a genetic disorder resulting in the absence of a vital enzyme called hexosaminidase A. This enzyme is necessary for the normal metabolism of a certain fatty substance. Without the enzyme, the substance builds up to toxic levels, particularly in the brain. At present no cure is known, and the disease leads to death.

Science has been able to control some other enzyme deficiencies that formerly were as catastrophic as Tay-Sachs. Phenylketonuria, or PKU for short, is one such condition. PKU was once a significant cause of mental retardation. It is caused by the absence of the enzyme phenylalanine hydroxylase, needed to metabolize phenylalanine, one of the essential amino acids, the building blocks of proteins. Lack of the enzyme results in toxic accumulations of a substance normally beneficial to the body.

Most states routinely screen infants for PKU. Once identified, this problem is controlled through careful nutrition. Phenylalanine is omitted from the diets, preventing any toxic build-up. Until science finds a permanent cure, this vigilance must be continued throughout the patient's life. It is particularly important for those with PKU to know that Nutra-Sweet (aspartame) contains phenylalanine. Products that contain aspartame must carry a warning to alert PKU patientS.

Enzyme deficiencies can be accidentally induced through poisoning. For example, heavy metals, such as lead and mercury, are toxic, in part because they inactivate vital enzyme systems.

Medical science has long used certain enzymes with beneficial results. Fibrinolytic enzymes--which help dissolve blood clots--have been used to clean wounds that have dried and clotted under unsanitary conditions. This job was once done by physical scraping or application of maggots to the wound. Some of these same enzymes are now being used experimentally to dissolve blood clots in coronary artieries immediately following heart attacks. It is hoped that the enzymes will safely dissolve the clots in time to prevent significant damage to the heart.

Enzymes are also used as an alternative to spinal surgery to repair the leakage of a ruptured disk. In this treatment, the material that has "leaked" out of the disk is dissolved by action of the enzyme chymopapain. While no surgery is required, very precise injection of the enzyme is necessary, and the procedure can involve serious risks. (See "Drug for Slipped disks" in the Updates section of the February 1983 FDA Consumer.)

Enzymes are also important indicators of body function. Two notable examples are known by their acronyms SGOT and SGPT (serum glutamic oxaloacetic transaminase and serum glutamic pyruvic transaminase). Destruction of certain body tissues due to injury results in release of large quantities of SGOT and SGPT into the bloodstream, generally proportionate to the damage. Measurement of SGOT and SGPT levels thus provides an estimate of the extent of the injury, which helps doctors plan proper treatment.

From digesting foods to metabolizing drugs, enzymes are a vital part of our bodies. And with genetic engineering and recombinant DNA technologies currently under development, it seems likely that many of the recognized enzyme deficiency diseases will become controllable if not curable.

COPYRIGHT 1986 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group