Ptyalin is a protein found in saliva that breaks down the insoluble starch found in foods into smaller, soluble sugar fragments. This is another name for the amylase found in saliva, known as salivary amylase. This particular type of amylase is found only in humans.
Foods such as potatoes and rice contain a lot of starch, a carbohydrate. Starch is an insoluble polysaccharide — a long chain of sugars. The sugar in this case is glucose, and there are branches coming off of the chain.
An important component of digesting food is breaking the food into smaller pieces that can be more easily digested. Part of this is accomplished physically by chewing, but much of it is accomplished by the actions of enzymes. These are proteins that speed up reactions, making them chemically feasible.
The initial act of chewing food stimulates the three sets of salivary glands to produce saliva. The function of these glands is to make enzymes. They have a duct that connects them to the mouth, and has a protective barrier to keep the enzymes from digesting the tissue there.
The saliva contains ptyalin, which starts degrading the starch where the chain is linear. It is an enzyme that breaks the bond joining two sugars together, and forms fragments of soluble starches. It then breaks these down further, and the final product is a unit of two glucose molecules, known as maltose. This substance is then broken down to glucose in the intestine.
Ptyalin does not have much time to act in the mouth, however. Fairly quickly, the tongue masses the food and saliva together as a bolus in preparation for swallowing. The protein continues its maltose production within the bolus, even after it has reached the stomach.
The stomach is highly acidic, and these conditions are very unfavorable for ptyalin, which prefers a high pH for activity. The starch and glucose provide some protection for the enzyme from the stomach’s acid, and it continues degrading starch for a short period of time until it is inactivated. Overall, the ptyalin only degrades a small amount of the total starch in the diet. The rest is degraded by a pancreatic amylase.
The amount of this protein differs in people of different ethnic backgrounds. This was discovered through genetic research. It is thought to be due to the amount of starch consumed by a people. For instance, the traditional Japanese diet contains much more starch than that of hunter-gatherers in forest areas of Europe. Correspondingly, the Japanese typically have higher amount of the enzyme than some Europeans.
Scientifically speaking, ptyalin is a metalloenzyme, and requires a calcium ion to function. It is an alpha-amylase. This type of amylase can cleave anywhere in the linear part of the starch chain. It is much more efficient at cleavage than a beta-amylase, which repeatedly cleaves off a single unit of maltose from the end of the polysaccharide. Humans do not produce beta-amylase.