Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 8 Porphyrins and the Porphyrias
porphyrin isomers as proceeding along parallel and independent
paths.
The uroporphyrins also contain two different radicals,
acetic (A) and propionic (P) acids, and four each of these
are arranged to correspond to either isomer ETIO I or ETIO
III ( Figure 8-2 ). In this case, A corresponds to M and P
8 H
7 H
D
C
C
1 2
H H H
NH
H
6
A
N
N
C
HN
H
5
C
C
B
H 3
H 4
Porphin
8
7
D
1
H
A
C
6 5
Pyrrole
FIGURE 8-1 The precursor pyrrole and the parent porphin nucleus of
porphyrins. Sites of isomeric substitutions are given as circled numbers
and the pyrrole rings as letters. A schematic representation is also given.
2
N
H
B
3
4
H
H
corresponds to E. Therefore, these are designated uroporphyrin
I (URO I) or uroporphyrin III (URO III). Similarly,
the coproporphyrins contain four M and four P groups and
are designated coproporphyrin I (COPRO I) and coproporphyrin
III (COPRO III). The protoporphyrin of heme
(iron-protoporphyrin, the prosthetic group of hemoglobin)
corresponds to the series III isomer. In this case, however,
three different radicals instead of two are substituted.
These consist of four M, two P, and two vinyl (V) radicals.
With three different radicals, 15 isomers are possible, but
the protoporphyrin of heme is the only naturally occurring
isomer known. This isomer was designated protoporphyrin
IX because it was the ninth in the series of protoporphyrin
isomers synthesized by Fischer. The arrangement of the
methyl groups of this isomer as shown in Figure 8-2 corresponds
to that of a type III etioporphyrin isomer and more
properly should be called protoporphyrin III. However, by
convention, the name protoporphyrin IX (PROTO IX) is
the designation for this porphyrin.
An interesting isomer with only one vinyl group
located on the A ring occurs and is known as harderoporphyrinogen,
which is thought to be an intermediate in the
synthesis of protoporphyrinogen III. Other naturally occurring
or chemically synthesized porphyrins are derivatives
of PROTO IX. If the two vinyl groups are hydrogenated
to ethyl groups, the product is mesoporphyrin IX. If the
M E
A
E
M
D ETIO I B
M
E
C
E M
M
E
M E
ETIO II
E M
E
M
M
E
M E
ETIO III
E M
M
E
E
M
M E
ETIO IV
M E
E
M
FIGURE 8-2 The isomeric porphyrins. The
nomenclature of the porphyrins URO, COPRO,
and PROTO is based on the isomeric structure
of the etioporphyrins. Note that there are only
the type I and type III isomers.
A
P
A
P
P
A
URO I
A
P
A
A
URO III
A
P
P
A
P
A
M
P
M
P
P
M
COPRO I
M
P
M
P
COPRO III
M
P
P
M
P
M
M Methyl
E Ethyl
A Acetic acid
P Propionic acid
V Vinyl
M
V
M
PROTO III (9)
P
M
V
P
M