ESR study of 5'-nucleotidase from bull seminal plasma.

5'-Nucleotidase of bull seminal plasma has been spin labeled with the sulfhydryl reagent 3-maleimidoproxyl. ESR analysis reveals the presence of two classes of labeled sites. The first is characterized by a long spin label rotational correlation time, from which a protein diameter of about 70 A can be estimated, under the assumption of a spherical shape. The second class is characterized by a shorter correlation time of the covalently bound spin labels and binding of the substrate sodium thymidine 5'-monophosphate to 5'-nucleotidase results in a reduction of their mobility. Low-temperature ESR analysis shows that no paramagnetic ion is bound to the native protein.