Use of cysteine as a spectroscopic probe for determination of heme-scavenging capacity of serum proteins and whole human serum.

•Detection of free extracellular heme is challenging issue.•Cysteine can be used as an absorbance spectroscopy probe for distinguishing protein-bound from free heme.•Use of cysteine allow detection of heme bound to proteins with low affinity.•The strategy might be used for assessment of heme-binding capacity of plasma or serum of patients with hemolytic diseases.