Proteins

RUTH M. LEVERTON.

YOU are looking at a superb package of proteins when you see yourself in a mirror. All that shows muscles, skin, hair, nails, eyes are protein tissues. Teeth contain a little protein.

Most of what you do not see is protein, too blood and lymph, heart and lungs, tendons and ligaments, brain and nerves, and all the rest of you.

Genes, those mysterious controllers of heredity, are a particular kind of protein.

Hormones, the chemical regulators of body processes, and enzymes, the sparkplugs of chemical reactions, also are proteins.

Life requires protein. A Dutch physician turned chemist, Gerrit Jan Mulder, first observed this fact, which we now take for granted. He announced in 1838 his conclusion from many investigations that all living plants and animals contain a certain substance without which life is impossible. He did not know what was in it, but he was sure it was vital. He named it protein, from a Greek word meaning first place.

Scientists since then have discovered that there are hundreds of different kinds of protein not just the one sub-stance Mulder observed. They also learned that proteins are unique in that they contain the element nitrogen. All our foodstuffs fats, starches, sugars, and proteins contain the elements carbon, hydrogen, and oxygen in varying proportions. Because proteins contain them and also nitrogen, they have a special importance and power.

Proteins have to be made by living cells. Proteins do not exist in air, like nitrogen or oxygen. They do not come directly from the sun, like energy.

Most plants make their own protein by combining the nitrogen from nitrogen-containing materials in the soil with carbon dioxide from the air and with water. The energy they need for the process comes from the sun. Legumes, which include beans, peas, and peanuts, can use the nitrogen directly from the air for combining with the other substances to make protein.

Animals and people cannot use such simple raw materials for building the proteins. We must get our proteins from plants and other animals. Once eaten, these proteins are digested into smaller units and rearranged to form the many special and distinct proteins we need.

Although we sometimes hear plant proteins referred to as "inferior" to animal proteins, plants are the basic factory of proteins. All proteins come directly or indirectly from plants.

We depend heavily on farm animals to convert plant proteins into animal proteins for us, but most animals, too, must have some animal protein supplied to them. The ruminant animals cattle, sheep, goats are an exception, because they can use the simple nitrogen-containing substance in young pasture grasses; the micro-organisms in their paunches can make microbial proteins, which the animal can then digest and use.

NEXT TO WATER, protein is the most plentiful substance in the body. If all the water were squeezed out of you, about half your dry weight would be protein. About a third of the protein is in the muscle. About a fifth is in the bone and cartilage. About a tenth is in the skin. The rest is in the other tissues and body fluids. Bile and urine are the only fluids that do not contain protein.

There are several dozen proteins in the blood alone.

One of the busiest is hemoglobin, which constantly transports oxygen from the lungs to the tissues and brings carbon dioxide back from the tissues to the lungs. Ninety-five percent of the hemoglobin molecule is protein. The other 5 percent is the portion that contains the iron.

Other proteins in the blood are defenders, for they give us the means of developing resistance and sometimes immunity to disease.

Gamma globulin can also form antibodies, substances that can neutralize bacteria and viruses and other micro-organisms. Different antibodies are specific for different diseases.

Once we have had a disease, like measles, and the antibody for measles has been formed, it stays in the blood, and we are not likely to have measles again. A vaccination, such as for poliomyelitis, introduces a tiny amount of the inactive or dead virus into the body to stimulate the blood to form the specific antibody needed for neutralizing the virus that causes poliomyelitis. The presence of an antibody in the blood may give the person immunity to the disease. At least it gives him a head start in fighting the virus, and the disease will be less severe.

Gamma globulin also helps the scavenger cells phagocytes engulf disease microbes.

Proteins help in the exchange of nutrients between cells and the intercellular fluids and between tissues and blood and lymph. When one has too little protein, the fluid balance of the body is upset, so that the tissues hold abnormal amounts of liquid and become swollen.

The proteins in the body tissues are not there as fixed, unchanging substances deposited for a lifetime of use. They are in a constant state of exchange. Some molecules or parts of molecules always are breaking down and others are being built as replacements. This exchange is a basic characteristic of living things; in the body it is referred to as the dynamic state of body constituents the opposite of a static or fixed state. This constant turnover explains why our diet must supply adequate protein daily even when we no longer need it for growth. The turnover of protein is faster within the cells of a tissue (intracellular) than in the substance between the cells (intercellular).

Proteins, like starches, sugars, and fats, can supply energy.

One gram of protein will yield about 4 Calories when it is combined with oxygen in the body. One ounce will give 115 Calories. That is about the same amount as starches and sugars give.

The body puts its need for energy above every other need. It will ignore the special functions of protein if it needs energy and no other source is available. This applies to protein coming into the body in food and to protein being withdrawn from the tissues. Either kind gets whisked through the liver to rid it of its nitrogen and then is oxidized for energy without having a chance to do any of the jobs it is especially designed to do. The protein-sparing action of carbohydrates means that starches and sugars, by supplying energy, conserve protein for its special functions.

WE CANNOT TALK about proteins very long without getting into the subject of the amino acids, the chemical units of which proteins are made. I discuss them in the next chapter, but I must point out here that the kinds and amounts of amino acids in a protein determine its nutritive or biological value.

The amino acid composition of animal muscle, milk, and egg is similar, though not identical, to the amino acid composition of human tissues. Because these animal proteins can supply all of the amino acids in about the same proportions in which they are needed by the body, they are rated as having a high nutritive value.

The proteins from fruits, vegetables, grains, and nuts supply important amounts of many amino acids, but they do not supply as good an assortment as animal proteins do. Their nutritive value therefore is lower. The proteins from some of the legumes especially soybeans and chickpeas are almost as good as those from animal sources.

To have the nutritive value of the mixture of proteins in our diets rate high requires only that a portion of the protein come from animal sources.

To STUDY THE NEEDS of people and animals for proteins, scientists commonly study the nitrogen balance.

Nitrogen is easier to measure than Protein. The amount of nitrogen, properly determined, is an accurate index of the amount of protein involved. Because the common proteins average 16 percent nitrogen, we can measure the amount of nitrogen in a food, multiply the amount by 6.25, and get our answer in grams of protein.

A nitrogen-balance study is based on the principle that if we know the amount of nitrogen that goes into the body in the food and the amount that leaves the body in the excreta, we can calculate what has been used. The amount that has been used reflects the amount that has been needed.

The body constantly uses materials for maintenance, regardless of the supply. It operates best when the supply of materials from food is generous and regular, but it does not stop functioning immediately when the food fails to supply what is needed. It mobilizes material from its tissues to meet these needs as long as that supply will last.