Lactase - Enzymes
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--- Lactase ---

The use of a chemical analog as a enzyme's substrate.

.....You have heard about lactose intolerance. Such people must either not consume any lactose (milk sugar), or artificially break it down in the milk, ice cream or cheese before they eat those things. Do you know anyone who is lactose intolerant?

.....Let's look at the human ingestion of lactose. Back in cave-days, the only time a person would ever ingest lactose would be when they were infants and getting milk from their mothers. Thereafter in their lives milk was never consumed. Only with the invention of agriculture and animal husbandry has milk become readily available to adults. Do you know of any cultures that still rarely have milk products available to adults? Did you ever get ice cream in a Chinese restaurant? Many people of Oriental or African descent are extremely lactose intolerant as adults. They often get violently nauseated upon eating a spoonful of lactose. As a matter of fact, most of you would feel a bit strange after a tablespoon of lactose. You probably aren't violently lactose intolerant, but actually lactose tolerant, but still not lactose degraders. Babies are lactose degraders because in that period of their development genes are turned on that lead to the production of lactase - the enzyme that splits the disaccharide we call lactose.

A figure showing a lactose molecule's being hydrolyzed by the enzyme lactase into two monosaccharides - glucose and galactose.

.....Lactose is a disaccharide with one glucose sugar molecule bound to one galactose sugar molecule. Once lactose is split, our bodies readily metabolize the glucose and galactose products. Now, can you think of any other developmental stage in which a person produces lots of lactase? When else in a human's lifetime is there lots of lactose in the body? The nursing mother, of course, makes lactose to put into her milk to feed her baby. She uses lactase to catalyze the reverse reaction: glc + gal → lactose. Later, the baby takes it in the opposite direction: splits the lactose, and the glucose mostly is metabolized for energy, and some of the galactose goes into making brain material. Hence, generally the more intelligent the mammal, the more lactose in mother's milk.


TRIVIA:The word "mammal" derives from "mamma" or "ma",
which are among the very, very few universal words.


.....Let's watch lactase work. Alas, it is hard to observe lactase work on lactose, unless you want to taste the concoction to see if it gets slightly sweeter with time. Lactose is barely sweet. Galactose is a just a tad sweeter; and glucose is about half as sweet as table sugar (sucrose). (But people frown on tasting things in the lab because we never know what was in the beaker or flask previously.) But there is another way to see lactase work. Suppose we had a chemical that was somewhat similar to lactose, but that when split, one portion becomes a colorful dye. We thus wish to use an ANALOG of lactose. A commonly used one is ONPG (ortho-nitro-phenyl-galactoside). This analog has a "ONP" instead of the monosaccharide glucose. When the bond is snipped between the ONP and the Galactose, the resulting ONP is a bright yellow water-soluble dye. Thus, as lactase works on zillions of ONPG molecules in a solution, the solution slowly turns yellow as more and more of the ONPG molecules are snipped in half.

A figure of ONPG's being hydrolyzed by lactase into two molecules - galactose and a yellow ONP.

....."QUALITATIVE" EXPERIMENT:.....Break off a very small piece of a lactaideŽ pill. Crush it and drop it into a test tube and add about a half inch of water; swirl to dissolve. Add six drops of the ONPG solution. Note how fast the solution turns yellow. (Do not taste this solution as the yellow ONP is toxic.) (Teachers: This must be made fresh for each lab. ONPG solution: Take out a bottle of ONPG powder from freezer; allow it to warm before opening so as to prevent condensation from forming INSIDE the bottle when you open it. Weigh out 80 mg and dissolve in 10 ml of water. Dispense into several small tubes for student use.)


| An experiment on feed-back inhibition of LACTASE activity |


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