Determination of antioxidant capacity in human serum using the Agilent Cary Eclipse for the ORAC assay

Significance of the topic

Reactive oxygen species (ROS) play a dual role in biological systems, driving essential processes such as ATP production while posing a risk of oxidative damage when their levels become imbalanced. Monitoring total antioxidant capacity in serum offers a stable and integrative measure of the body’s defense against ROS, which is linked to chronic diseases including Alzheimer’s, cardiovascular disorders, and diabetes.

Objectives and study overview

This application note demonstrates how to implement the oxygen radical absorbance capacity (ORAC) assay on the Agilent Cary Eclipse fluorescence spectrophotometer with a temperature-controlled multicell accessory. The goal is to quantify serum antioxidant capacity and compare results to established methods.

Methodology and instrumentation

The ORAC assay evaluates antioxidant activity by monitoring fluorescence decay of R-phycoerythrin (R-PE) under free radical attack by AAPH. Key steps include:

• Preparation of human serum (diluted 1:100 in phosphate-buffered saline, PBS).
• Use of R-PE as a fluorescent probe and AAPH as the radical generator.
• Trolox standards for calibration and blank controls.
• Fluorescence kinetics recorded at optimal excitation/emission wavelengths, with integration of the area under the decay curve.

Instrumentation:

• Agilent Cary Eclipse fluorescence spectrophotometer
• Peltier-thermostatted multicell holder with electronic stirring
• Temperature controller and probes
• Quartz cuvettes and magnetic stir bars

Key results and discussion

Fluorescence decay curves clearly distinguished blank, trolox standard, and serum samples. The calculated ORAC value for human serum was 8.23 mM, compared to 3.1 mM reported by Cao and Prior. The higher value may reflect the younger, non-fasted volunteer cohort and the synergistic action of multiple serum antioxidants, which prolong free radical scavenging.

Benefits and practical applications

The Agilent Cary Eclipse platform provides precise temperature control (±0.2 °C) and uniform stirring across four cuvettes, ensuring reproducible ORAC measurements. Its sensitivity and scanning capability enable lower reagent use and adaptability to novel redox-sensitive probes, making it ideal for high-throughput antioxidant screening in clinical and research laboratories.

Future trends and applications

Opportunities for enhancing the ORAC assay include:

• Increasing AAPH concentration or further diluting samples to extend reaction times and throughput.
• Adapting protocols to emerging fluorescent probes with distinct spectral properties.
• Automating sample handling for large-scale antioxidant capacity profiling in clinical studies.

Conclusion

The ORAC assay on the Agilent Cary Eclipse with a Peltier-controlled multicell accessory delivers accurate, reproducible measurements of serum antioxidant capacity. Its combination of sensitivity, temperature stability, and flexible scanning makes it a valuable tool for oxidative stress research and quality control applications.

Reference

1. Halliwell B, Gutteridge JMC. Free radicals in biology and medicine. 3rd ed. Oxford: Clarendon Press; 1999.
2. Cao G, Prior RL. Comparison of different analytical methods for assessing total antioxidant capacity of human serum. Clinical Chemistry. 1998;44(6):1309–1315.
3. Benzie IFF, Strain JJ. The ferric reducing ability of plasma (FRAP) as a measure of ‘antioxidant power’: The FRAP assay. Analytical Biochemistry. 1996;239:70–6.
4. Rice-Evans C, Miller NJ. Total antioxidant status in plasma and body fluids. Methods in Enzymology. 1994;234:279–293.
5. Cao G, Prior RL. The measurement of oxygen radical absorbance capacity in biological samples. Methods in Enzymology. 1999;299:50–62.
6. Saucier CT, Waterhouse AL. Synergetic activity of catechin and other antioxidants. Journal of Agricultural and Food Chemistry. 1999;47(11):4491–494.