January 20, 2009
To the Editor:
McDonagh et al.(1) suggest that bilirubin photoisomers may interfere
unpredictably with measurement of plasma unbound bilirubin concentration
(Bf) by the peroxidase test in jaundiced newborns. Following in vitro
conversion of neurotoxic 4Z,15Z bilirubin-IXα to photoisomers in a
defatted human albumin solution (HSA) and commercial adult sera (CAS), the
decrease in Bf was unrelated to the decrease in ZZ-bilirubin
concentration. We urge caution in extrapolating their results to
jaundiced newborns.
The Bf of 8 µg/dL (0.14 µmol/L) in HSA is much higher than Bf
occurring in jaundiced newborns with similar bilirubin/albumin molar
ratios and higher than Bf in a case of lethal kernicterus.(2) Bilirubin
solubility and self-aggregation may have compromised the accuracy of Bf
measurements in this sample. Itoh et al.(3)using HSA and a clinically
relevant Bf of 0.61 µg/dL found photoisomers actually increased Bf, but
only after Z-lumirubin reached levels well above those of McDonagh et
al.(1) Although photoisomers bind to proteins less avidly than ZZ-
bilirubin, they are oxidized less readily to products with greater
absorbances than ZZ-bilirubin, limiting their impact on the peroxidase
test.(3)
The ratio of Z-lumirubin to ZE isomer (0.43) in CAS was much higher
than that in the HSA sample (0.13) as well as that typically seen in
newborn sera (≈ 0.15).(3,4) The atypical photoisomer pattern
suggests the structural orientation of bound ZZ-bilirubin in CAS is not
comparable to that in HSA or newborn sera. Therefore CAS binding of
bilirubin may be intrinsically different from bilirubin binding in newborn
sera.
McDonagh et al.(1)emphasize the disproportionate percentage changes
in Bf versus total bilirubin following phototherapy as “unexpected” and
evidence photoisomers may muddle the peroxidase test. However, the law of
mass action predicts a disproportionate change, and phototherapy did not
greatly alter bilirubin binding constants in the clinical studies
cited.(5) Whether the peroxidase test over- or underestimates neurotoxic
Bf (or total bilirubin) in the presence of photoisomers will remain
uncertain until the neurotoxicity of ZE-bilirubin is determined.
The data of McDonagh et al.(1)are not compelling evidence that
bilirubin photoisomers may interfere in an unpredictable manner with the
correlation between peroxidase Bf measurements and bilirubin neurotoxicity
in jaundiced newborns. In the clinical setting, both total and unbound
bilirubin are compromised to the extent that non-toxic bilirubin isomers
contribute to their measurements. If Bf is a better predictor of
bilirubin neurotoxicity than total bilirubin, phototherapy is unlikely to
undermine that superiority in clinical studies.
Charles E. Ahlfors, MD, Seattle, WA, USA
Richard P. Wennberg, MD, Seattle, WA, USA
J. Donald Ostrow, MD, Seattle, WA, USA
Claudio Tiribelli, MD, PhD, Trieste, Italy
References
1. McDonagh AF, Vreman HJ, Wong RJ, Stevenson DK. Photoisomers:
Obfuscating factors in the peroxidase measurement of unbound bilirubin?
Pediatrics. 2009; 123:67-76
2. Ahlfors CE, Herbsman O. Unbound bilirubin in a term newborn with
kernicterus. Pediatrics 2003;111:1110-1112
3. Itoh S, Yamakawa T, Onishi S, Isobe K, Manabe M, Sasaki K. The
effect of bilirubin photoisomers on unbound-bilirubin concentrations
estimated by the peroxidase method. Biochem J. 1986; 239(2):417–421.
4. Myara A, Sender A, Valette V, et al. Early changes in cutaneous
bilirubin and serum bilirubin isomers during intensive phototherapy of
jaundiced neonates with blue and green light. Biol Neonate 1997;71:75-82
5. Ebbesen F, Jacobsen J. Bilirubin-albumin binding affinity and
serum albumin concentration during intensive phototherapy (blue double
light) in jaundiced newborn infants. Eur J Pediatr. 1980; 134:261-263.
Conflict of Interest:
None declared
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