SECTION 6.4
Inhibition
97
4-Methylpyrazole (fomepizole), a more effective inhibitor
of alcohol dehydrogenase with none of the adverse effects
of ethanol, has been used in the treatment of ethylene
glycol toxicity.
Isopropanol, present in rubbing alcohol, hand lotions,
and antifreeze preparations can be ingested accidentally
by young children and intentionally by alcoholics or sui-
cidal adults. Isopropanol is oxidized to acetone, a toxic,
nonmetabolizable product, by alcohol dehydrogenase. It
should be noted that acetone is also present in diabetic ke-
toacidosis (Chapter 22). However, diabetic ketoacidosis is
accompanied by glucosuria, whereas isopropanol inges-
tion shows the presence of acetone without glucose. Tox-
icity of isopropanol includes profound CNS depression,
with coma as the most common presentation, gastritis,
vomiting, and hemorrhage are also common. Acetone is
also a CNS depressant and can be detected in the urine.
The treatment of isopropanol toxicity is supportive and is
removed by hemodialysis.
In evaluating individuals for toxicity to substances such
as methanol, ethylene glycol, and isopropanol, the fol-
lowing serum components are measured: Na+, K+, Cl- ,
HCOf, glucose, urea nitrogen, blood osmolality, blood
gases, and urine ketones. The severity of the osmolality
gap, anion gap, and metabolic acidosis (Chapter 39), along
with pertinent clinical history, will aid in the identifica-
tion of the offending substance. In particular, the assess-
ment of osmolality gap, which represents the difference
between the measured serum osmolality and the calculated
osmolality is useful in the diagnosis of low-molecular-
weight toxic substances. Elevated serum levels of low-
molecular-weight toxic chemicals contribute to serum
osmolality significantly.
Serum osmolal gap = measured osmolality
—
calculated osmolality
Calculated osmolality = 2 x Na+ (mM/L)
+ glucose (mM/L)
+ urea nitrogen (mM/L).
In
noncompetitive inhibition,
the inhibitor does not usu-
ally bear any structural resemblance to the substrate, and
it binds to the enzyme at a site distinct from the substrate
binding site. No competition exists between the inhibitor
and the substrate, and the inhibition cannot be overcome by
increase of substrate concentration. An inhibitor may bind
either to a free enzyme or to an enzyme-substrate complex;
in both cases, the complex is catalytically inactive:
E + I ^ El (inactive)
ES + I ^ ESI (inactive)
The value of Vmax is reduced by the inhibitor, but
Km
is
unaffected because the affinity of S for E is unchanged.
Thus, comparison of the double-reciprocal plots, with and
without the presence of inhibitor (Figure
6
-
6
), shows that
both the slope and the intercept on the
y
axis (
1
/ Vmax) are
altered, whereas the
x
intercept (—
l/Km)
is unchanged.
Examples of noncompetitive inhibition are
1. Enzymes with sulfhydryl groups (-SH) that
participate in maintenance of the three-dimensional
conformation of the molecule are noncompetitively
inhibited by heavy metal ions (e.g., Ag+, Pb2+, and
Hg2+):
E-SH + Hg2+
E-S-Hg+ + H+
Lead poisoning causes anemia (low levels of
hemoglobin) owing to inhibition of heme synthesis at
two sites at least: porphobilinogen synthase and
ferrochelatase, both of which contain sulfhydryl
groups (Chapter 29). Heavy metal ions react with
S-containing, O-containing, and N-containing
ligands, present as -OH, -COO- , -OPO
3
H- , -C = 0 ,
-NH
2
, and -NH groups.
2. Enzymes that are dependent on divalent metal ion
(e.g., Mg2+ and Ca2+) for activity are inhibited by
chelating agents (e.g., ethylenediamine tetraacetate)
that remove the metal ion from the enzyme.
3. Enolase catalyzes a step in the metabolism of glucose
(see the discussion of glycolysis in Chapter 13),
2-Phospho-D-glycerate ^ phosphoenolpyruvate + H20
a reaction that has an absolute requirement for a
divalent metal ions (e.g., Mg2+ or Mn2+) complexed
to the enzyme before the substrate is bound. This
reaction is inhibited by fluoride ion (F- ) in a process
involving the formation of a complex with phosphate
giving rise to a phosphofluoridate ion that binds
magnesium ions. Thus, addition of fluoride ions
inhibits the breakdown of glucose in the glycolytic
pathway. For this reason, F- is used as a preservative
in clinical specimens (e.g., blood) in which glucose
determinations are to be made.
In
uncompetitive inhibition,
inhibitor, I,
combines with ES to form an enzyme-substrate-
inhibitor complex:
ES + I ^ ESI
The double-reciprocal plot with the inhibitor yields
parallel lines (i.e., the slope remains constant), but the
intercepts on both the x and
y
axes are altered by the
presence of the inhibitor (Figure
6
-
6
). The apparent
Vmax and the apparent
Km
are both divided by a factor
