Decarboxylation of uroporphyrinogen III by erythrocyte uroporphyrinogen decarboxylase. Evidence for a random decarboxylation mechanism

Abstract

The isomeric composition of type-III heptacarboxylic porphyrinogens derived from decarbosylation of uroporphyrinogen III by erythrocyte uroporphyringogen decarboxylase was analysed by h.p.l.c. with electrochemical detection. All four possible isomers were identified, and there were little differences in the proportion of isomers formed by erythrocytes from normal subjects and from patients with sporadic porphyria cutanea tarda. The results provide conclusive evidence that the normal decarboxylation pathway is random in nature, and the fourth isomer only increases when enzyme abnormality is found.

Purification and properties of the uroporphyrinogen decarboxylase from Rhodobacter sphaeroides

“…The intermediates found to accumulate were all of the D-ring isomer (Jackson et al, 1976), such as 7111D in Figure 1, which have been shown to act as excellent substrates for the enzyme. In contrast, studies with the human erythrocyte decarboxylase suggested that the decarboxylation follows a random route (Luo and Lim, 1990).…”

“…These studies led to the conclusion that the decarboxylation from uroporphyrinogen III to coproporphyrinogen III proceeds by a preferred route, commencing with the decarboxylation of the acetic acid group on ring D followed by those on rings A, B and finally C (Jackson et al, 1976). Recent studies with normal human erythrocytes suggest that the decarboxylation reactions catalysed by this enzyme may proceed by a random route (Luo and Lim, 1990) in which the decarboxylation of all four type-III isomers of the heptacarboxylic acid porphyrinogens (Figure 1) are produced. From these experiments it may be deduced that uroporphyrinogen decarboxylase has a relatively broad substrate specificity and can recognize uroporphyrinogen III and all 13 intermediate type-III porphyrinogens en route to coproporphyrinogen III.…”

Action of uroporphyrinogen decarboxylase on uroporphyrinogen-III: a reassessment of the clockwise decarboxylation hypothesis

“…Indeed, there was no indication of a preferred decarboxylation pathway for uroporphyrinogen-ITI, hepta-d, or hexa-da. Recently, a similar study was carried out for human erythrocyte uroporphyrinogen decarboxylase (Luo & Lim, 1990) and uroporphyrinogen-Ill was shown to form all four possible heptas in roughly equal portions. These authors suggest that their work 'provides conclusive evidence for a random, rather than ordered, decarboxylation sequence'.…”

“…In each case, they demonstrated that all fourteen of the possible hepta, hexa and penta isomers were present in normal urine in similar proportions. However, they were able to confirm that the major hepta in the urine of PCT patients was hepta-d (Luo & Lim, 1990). In addition, h.p.l.c.…”

“…analysis. On the other hand, all 14 intermediates were detected' in normal' human urine (Lim et al, 1983) and incubation of uroporphyrinogen III with red cell haemolysates produced a mixture of isomers (Lash, 1979;Luo & Lim, 1990), indicating random decarboxylation. Lash (1991) postulates that the uroporphyrinogen III used in the enzyme reaction may be presented randomly to the enzyme leading to random decarboxylation while with PBG as substrate the enzymically produced uroporphyrinogen III is 'handed on' to uroporphyrinogen decarboxylase in, a specific orientation, resulting in specific decarboxylation of the ring-D acetate group.…”