Wavelength-dependence of the relative rate constants for the main geometric and structural photoisomerization of bilirubin IX α bound to human serum albumin. Demonstration of green light at 510 nm as the most effective wavelength in photochemical changes from (ZZ)-bilirubin IX α to (EZ)-cyclobilirubin IX α via (EZ)-bilirubin

Abstract

The kinetics for the quantitatively important reaction: (Formula: see text) that is, the photochemical interconversion between bilirubin and its geometric and structural photoisomers bound to human serum albumin in aqueous solution when various wavelengths of monochromatic light were used, were assayed by h.p.l.c. In order to clarify the wavelength-dependence of the relative rate constants in the individual steps, a light-source with a half-bandwidth of 10 nm was used at increments of 20 nm, in the range from 410 nm to 550 nm. We describe for the first time studies on the wavelength-dependence of rate constants in geometric and structural photoisomerization reactions in vitro of (ZZ)-bilirubin or (EZ)-bilirubin bound to human serum albumin, especially the relative rate constants of cyclization of (EZ)-bilirubin into (EZ)-cyclobilirubin. Because studies in vitro have demonstrated that the wavelengths from 350 to 450 nm are mutagenic, the results obtained indicated that the safest and ideal light-source for phototherapy is green light of 510 nm, which keeps (ZE)-bilirubin concentrations as low as possible, as shown by a maximal value of k2 at 510 nm and a relatively low value of k1 at 510 nm. This light-source still ensures the substantial absorption of (ZZ)-bilirubin, which is the precursor of (EZ)-bilirubin, the intermediate in (EZ)-cyclobilirubin formation and, furthermore, as shown by the maximal value of k5 and a considerable value of k4 at 510 nm, promotes the cyclization of (EZ)-bilirubin derived from (ZZ)-bilirubin even though k3 at 510 nm also shows a peak value.

Bilirubin isomer distribution in jaundiced neonates during phototherapy with LED light centered at 497 nm (turquoise) vs. 459 nm (blue)

“…In this study, we found that the concentrations of E,Zbilirubin were much lower than those of Z,E-bilirubin, but in contrast to Z,E-bilirubin, it was highest for the group treated McDonagh et al (3,4) propose that E,Z-lumirubin is formed from both Z,Z-and E,Z-bilirubin, whereas Onishi et al (5) propose that E,Z-bilirubin is an obligate intermediate in the formation of E,Z-lumirubin. The serum concentrations of E,E-lumirubin were too low to be identified and quantified with certainty and therefore, are not reported.…”

“…However, it was difficult to detect the presence of a small difference of E,Z-lumirubin between the two treatments, due to the low concentrations. In vitro studies have shown, that the formation of lumirubin increases at longer wavelengths of light (3,5,16).…”

Change of bilirubin photoisomers in the urine and serum before and after phototherapy compared with light source

“…The effect of phototherapy takes place via three possible routes: photo‐oxidation, configurational photoisomerization, and structural photoisomerization. The significance of (EZ)‐C in phototherapy for human neonatal hyperbilirubinemia is as follows: (i) the amounts of (EZ)‐C excreted in bile and urine are greater than the amounts of (ZZ)‐BR reverted from (ZE)‐BR 2 ; (ii) the half‐life of serum (ZE)‐BR is longer than that of serum (EZ)‐C 7 ; (iii) after serum (ZE)‐BR has been excreted in bile, it reverts to (ZZ)‐BR which is absorbed in the intestinal tract, but (EZ)‐C is not observed in the intestinal tract because of the (EZ)‐C polymerization; (iv) enhancement of the efficacy of phototherapy by an increase in the intensity of light energy can not be explained by a mechanism via (ZE)‐BR; and (v) the clinical effect of phototherapy using green light can only be verified by the cyclobilirubin pathway 8 . The clinical effect of phototherapy by the light source in vivo is generally assessed by measurement of serum total BR level.…”

Phototherapy for neonatal hyperbilirubinemia

“…Onishi et al . and Itoh et al . found that the most efficient wavelength was 500–520 nm in the green region.…”