CLINICAL CASES
155
A nsw ers
[16.1]
B. The culprit here is cyanide produced from acetonitrile. Cyanide
inhibits the electron transport chain of cytochrome oxidase.
[16.2]
E. Acetonitrile itself is not the toxicant but undergoes metabolism
and produces cyanide, which is the toxic agent here.
[16.3]
E. Gluconeogenesis requires ATP, which is in short supply, turning up
the catabolism of glucose to lactate in the absence of an intact electron
transport chain. ADP cannot be transported into the mitochondrion
because ATP, its antiporter partner, isn’t made by oxidative phos-
phorylation as a result of cyanide inhibition of cytochrome oxidase.
Metabolism of fatty acids and ketone bodies requires a functional
electron transport chain for their metabolism, and these possibilities
are also ruled out.
[16.4]
B. Increased oxygen competes with cyanide bound to cytochrome
oxidase displacing it. Nitrites bind to hemoglobin converting it to
methemoglobin, which binds cyanide more tightly than cyanohe-
moglobin and pulls cyanide from cyanohemoglobin to form
cyanomethemoglobin. Thiosulfate is used to displace cyanide from
cyanomethemoglobin to form thiocyanate, which can be excreted, a
happy ending for cyanide poisoning. ^-acetylcysteine is used for
acetaminophen toxicity and not cyanide toxicity.
[16.5]
A. Rotenone binds avidly to the flavoprotein NADH CoQ reductase,
complex I (also called NADH dehydrogenase). The central portion of
the rotenone structure resembles the isoalloxazine ring of the FMN
molecule, and when it binds to complex I, rotenone prevents the
transfer of electrons from NADH to coenzyme Q.
[16.6]
D. Inhibition of the electron transport chain shuts down the major
pathway of regenerating NAD+ from the NADH produced in inter-
mediary metabolism. This forces the cytosolic conversion of pyruvate
to lactate to regenerate NAD+ so that glycolysis can continue in the
absence of a functioning electron transport system.
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