CASE FILES: BIOCHEMISTRY
by muscle exceeds the rate at which the Cori cycle can operate, lactate accu-
mulates in the blood leading to lactic acidemia.
During a marathon run, there is constant stretching and tearing of the leg
muscle fibers each time the foot hits the ground. This constant shock causes
damage to the muscle cells, resulting in a release of cellular contents to the
extracellular matrix and the bloodstream. The concentrations of myoglobin,
which is in high concentration in slow twitch (red) muscle fibers, and K+,
which is concentrated in all cells, therefore rise in the blood. When the con-
centration of myoglobin increases above 0.5 to 1.5 mg/dL, it is excreted into
the urine. Normally myoglobin is not toxic to the kidney; however, when the
pH of the urine drops below 5.6, myoglobin undergoes oxidation to produce
hematin (porphyrin bound Fe3+), which is toxic to the kidney and can lead to
acute renal failure. This toxic effect is exacerbated when the urine is concen-
trated as a result of dehydration.
C O M P R E H E N SIO N Q U E ST IO N S
[21.1] During the course of a marathon race a runner expends a large amount
of energy and must use stored sources of fuel as well as oxygen.
Compared to the beginning of the race (first mile), which of the fol-
lowing best describes the utilization of glycogen and fatty acids as
fuels and amount of oxygen consumed after running for 26 miles?
Fatty acids used
[21.2] During an extended period of exercise, the enzymes involved in the
glycolytic pathway in muscle tissue are actively breaking down glucose
to provide the muscle energy. The liver, to maintain blood glucose lev-
els, is synthesizing glucose via the gluconeogenic pathway. Which of
the following enzymes involved in these pathways would be most likely
to exhibit Michaelis-Menten kinetics, that is, have a hyperbolic curve
when plotting substrate concentration versus velocity of the reaction?
C. Lactate dehydrogenase
D. Phosphofructokinase 1
E. Pyruvate kinase