Holy Spit!

I am a bit of a klutz. For example, my attempts to put on mascara usually end up something like this:


I am usually in a hurry in the morning, so my make up remover is a little spit on my finger. Sometimes I find it is just as good as soap!

Naturally, my curious mind started to wonder… what makes spit work so well as a cleaner? Is it just me, or does it really have some cleaning properties?

It turns out that spit has all sorts of cool properties, in addition to being a pretty cool cleaning agent.

First, it has antibacterial abilities that keep us safe from whatever might be living on our food when we eat it. This antibacterial capability can act in other ways, as well. When you see a dog licking its wounds, it is not only cleaning out debris; the saliva also acts as an antibacterial agent to prevent infection and promotes new growth. People have actually done research on this, which you can find at this link.

Saliva kills bacteria because it contains lysozymes, an enzyme that breaks down bacterial cell walls. It also has lactoferrin, a molecule which binds to iron, killing bacteria that need iron to survive. Lastly, saliva contains immunoglobulin A which binds to pathogens and triggers our immune system.

Second, saliva contains mucous, which gives it it’s lubricating abilities. This is very important for digesting food, speech, and other activities…. Read here to learn what life would be like with a deficiency in saliva and the research that is currently being done to treat this disease.

Third, saliva contains digesting agents. These enzymes start the process of digesting in the mouth as you chew, and are often present in the stomach as well. The two main enzymes include lingual lipase and ptyalin, or alpha-amylase. These enzymes are the reason we can digest food, as well as use spit to clean up our make up ūüôā

It turns out that I am not the only one that uses spit as a cleaning agent. At a museum in Cleveland, art conservators use cotton swabs moistened with their mouths to remove dirt from their art. They call it cleaning with a “mild, enzymatic solution.” For example, the painting “Oedipus at Colonus” by Fulchran-Jean Harriet was covered with cigarette smoke when it arrived at the Cleveland Museum of Art. Spit cleaning was decided as the best way to remove the stains while preserving the painting.¬†You can read more about their process here.

So how EXACTLY do these two digesting agents, lingual lipase and alpha-amylase work? Let’s learn a little biochemistry.

Lingual lipase is secreted by the salivary glands of the tongue and breaks down triglycerides, or fats, into monoglycerides and¬†fatty acids. You can read the paper¬†from 1973 that originally discovered and located the source of lingual lipase in rats. That’s right, they dissected teeny tiny rat tongues.

Rat tongue: Top view showing circumvallate papilla (CP), foliate papillae (FP), lateral oral pharyngeal gland (P), and epiglottis (E). Adapted from Hamosh and Scow (1973) J Clin Invest 52: 88-95
Rat tongue: Top view showing circumvallate
papilla (CP), foliate papillae (FP), lateral oral
pharyngeal gland (P), and epiglottis (E). Adapted from Hamosh and Scow (1973) J Clin Invest 52: 88-95

They found that this enzyme is secreted by the salivary glands and released near the circumvallate papilla of the tongue. They also measured the enzyme’s activity at different acidity, or pH levels. Why does this matter? You may know that the stomach is an acidic environment, while the mouth is generally neutral. The pH at which an enzyme is most active can indicate where it usually functions.

Check out this figure from their paper.

Effect of pH on breakdown of triglycerides. Adapted from Hamosh and Scow (1973) J Clin Invest 52: 88-95
Effect of pH on breakdown of triglycerides. Adapted from Hamosh and Scow (1973) J Clin Invest 52: 88-95

This graph shows the activity of the enzyme versus the pH. The higher up on the graph the line goes, the more the enzyme is breaking down triglycerides into free fatty acids. The line travels from high acidity (low pH) to low acidity (high pH). The highest point in the line is at about pH 5, which is acidic. Since this enzyme is most functional at an acidic pH, that suggests that it can function in the stomach.

The scientists performed further experiments to confirm that lingual lipase indeed does originate in the mouth, yet also continues to digest triglycerides in the stomach.

The alpha-amylase ptyalin (pronounced “TIE-uh-lin”) breaks down starch found in¬†foods such as potatoes and bread. It breaks down starches into smaller and smaller starches until they reduce down to maltose, which is digestible. Interestingly, the gene coding for ptyalin has undergone changes in time, depending on exposure to starch in diet, which you can read about in this paper published in 2007. For example, multiple copies of the ptyalin gene have been found in Japanese individuals, of whom starch is highly consumed in the form of rice. The Biaka, a group of people who live as hunter-gatherers in the rainforest and who do not regularly consume high amounts of starch, were found to have a lower number of gene copies. In the line graph below, people groups exposed to¬†low¬†starch diets are shown in black and gray, while orange and red indicate people¬†consuming high amounts of starch. The y-axis shows the proportion of the people studied, while the x-axis indicates the number of copies in the ptyalin gene. You can see that a greater proportion of people with¬†high starch diets have gene duplications.

Adapted from Perry et al. (2007) Nat Genet 39: 1256-60. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2377015/
Adapted from Perry et al. (2007) Nat Genet 39: 1256-60.

Interestingly, just as spit may be used to clean ancient paintings, the alpha-amylase found in saliva is used in laundry detergents to get out stains from foods high in starch. This company sells a variety of enzymes found in the digestive process to use as an additive to detergents, and sorts their products by the stain removal desired.

Keep in touch with my blog, as I head to Boston for Experimental Biology. I will be blogging on behalf of the American Society for Pharmacology and Experimental Therapeutics!

For more information about saliva and digestion, read “Anatomy, Function, and Evaluation of the Salivary Glands” by Holsinger and Bui.

Author: ilovebraaains

I am a neuroscientist using zebrafish to study mechanisms of neuroregeneration.

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