section
1.3
Measurement of pH
11
base, this ratio assists in neutralizing acid and in maintain-
ing a constant pH. The H P O ^/^P O J buffering system
plays a minor role in plasma because of the low concen-
trations of these ions, but it is important in raising the
plasma pH through the excretion of H
2
PO
4
via the kidney
(Chapter 39). In summary, hemoglobin absorbs a major
portion of the hydrogen ions produced by the dissocia-
tion of H
2
CO
3
generated by the hydration of CO
2
—the
most important buffer system in the blood. However, since
hemoglobin and carbonic anhydrase are present only in red
blood cells, the HCOj"/H
2
CC
>3
system in the plasma is an
indispensable intermediary in transporting the acid. Thus,
the principal method of CO
2
transport is in the form of
HCO
3
in blood plasma.
H,N
NHÎ
c
i
NH
(c h 2)3
HCNHÎ
I
3
COO-
Arginine
nh;
(CH2)3
HCNHÎ
I
3
coo-
Ornithine
NHt
I
3
CH2
I
coo-
Guanidinoacetate
1.3
Measurement of pH
S-adenosylmethionine -
S-adenosylhomocysteine.
Guanidinoacetate
Methyttransferase (GMT)
Blood and urine pH can be measured easily by means
of a calibrated glass electrode, whereas pH measurement
inside the metabolizing cells is not easily accomplished.
Techniques for estimating intracellular pH include glass
electrode measurements on homogenates, calorimetric or
fluorometric analysis of intracellular distribution of indi-
cator dyes, and microelectrode methods.
Nuclear Magnetic Resonance and
Magnetic Resonance Imaging
The noninvasive technique of nuclear magnetic resonance
(NMR) spectrometry has been used to measure the con-
centration of H+ and other selected ions in isolated cells
and tissues. NMR analysis is based on the principle that
some atomic nuclei behave like tiny bar magnets because
the spinning of charged nuclei generates a magnetic mo-
ment along the axis of the spin. If a nucleus with a magnetic
dipole (spin) is placed in an external magnetic field, it will
acquire an orientation aligned either with the applied field
(low-energy state) or against the applied field (high-energy
state). The former state is analogous to the way in which
a compass needle aligns itself with the earth’s magnetic
field. Thus, the nuclei, in the presence of an external mag-
netic field, can remain in either of two
unequal
energy
states. If the aligned nuclei are excited with electromag-
netic energy of the proper frequency, some of the nuclei
in the low-energy state (ground state) will be excited to
the high-energy state. Subsequent release of energy by
excited nuclei leads to relaxation back to the ground state
and completes the resonance cycle between the two energy
states.
The NMR spectrum is essentially a measure of the
emission of electromagnetic radiation associated with the
H-N*
HN-PO2'
2
\ /
3
h2n
n h 2
2 \ /
c
1
HNCH,
Creatine Kinase
1
HNCH,
1
3
CH,
!
*
CH2
ADP
ATP
1
coo-
coo-
Phosphocreatine
Creatine
Non Enzymatic
Creatinine
FIGURE 1-9
Pathway of creatine biosynthesis. In GMT deficiency, precursor
guanidinoacetate accumulates and the synthesis of creatine and
phosphocreatine is severely reduced. Creatinine, a nonmetabolizable end
product that is excreted by the renal system, is also diminished.
return of the nuclei from the high-energy state to the
low-energy state. Each atomic nucleus has a character-
istic spectrum of resonance absorption frequencies that
are influenced by the chemical environment surrounding
that nucleus and that appear as shifts in the resonance fre-
quency (known as
chemical shifts).
Thus, the chemical
shifts (expressed numerically in parts per million [ppm]
relative to a standard NMR signal or frequency) can be
used to distinguish different chemical compounds contain-
ing the same nuclei. NMR spectral features are correlated
with spectra of known structures to provide structural in-
formation that may permit identification of the molecule
