Organic Chemistry

Section 22.17 Disaccharides 945

LACTOSE INTOLERANCE

Lactase is an enzyme that specifically breaks the

linkage of lactose. Cats and dogs

lose their intestinal lactase when they become adults; they are

then no longer able to digest lactose. Consequently, when they are

fed milk or milk products, the undegraded lactose causes diges-

tive problems such as bloating, abdominal pain, and diarrhea.

These problems occur because only monosaccharides can pass

into the bloodstream, so lactose has to pass undigested into the

b-1,4¿-glycosidic

large intestine. When humans have stomach flu or other intestinal

disturbances, they can temporarily lose their lactase, thereby be-

coming lactose intolerant. Some humans lose their lactase perma-

nently as they mature. Approximately 10% of the adult Caucasian

population of the United States has lost its lactase. Lactose intol-

erance is much more common in people whose ancestors came

from nondairy-producing countries. For example, only 3% of

Danes but 97% of Thais, are lactose intolerant.

GALACTOSEMIA

After lactose is degraded into glucose and galac-

tose, the galactose must be converted into glucose

before it can be used by cells. Individuals who do not have the

enzyme that converts galactose into glucose have the genetic

disease known as galactosemia. Without this enzyme, galactose

accumulates in the bloodstream. This can cause mental retarda-

tion and even death in infants. Galactosemia is treated by ex-

cluding galactose from the diet.

react with methyl iodide at C-4. The glucose residue, on the other hand, was unable
to react with methyl iodide at C-4 because the OH group at that position was used to
form the acetal with galactose.

O

CH 2 OH

OH

OH

H

H

O

HO

H

H

2,3,4,6-tetra-O-methylgalactose 2,3,6-tri-O-methylglucose

H

CH 2 OH

OH

H

H

H

O

HO

H

O

CH 2 OCH 3

CH 3 O

OCH 3

H

H

O

CH 3 O

H

H

H

CH 2 OCH 3

CH 3 O

H

H

H

O

CH 3 O

H

CH 2 OCH 3

CH 3 O

OCH 3

H

H

O

CH 3 O

H

H

OH

CH 2 OCH 3

CH 3 O

H

H

H

O

CH 3 O

H

HO

OH

OH OCH 3

acetal

linkage hemiacetal

linkage

acetal is

hydrolyzed acetal is

hydrolyzed

reacted

with CH 3 I

did not react

with CH 3 I

CH 3 I

excess

Ag 2 O

H 2 O