Effect of phospholipids on thyroid 5'-nucleotidase.

Phospholipids were separated from 5'-nucleotidase in thyroid plasma membranes by Sephadex G-200 gel filtration. After removal of lipids 5'-nucleotidase was still active and reassociation of the enzyme with phospholipids had a little effect on the increase of enzyme activity. Arrhenius plot of the 5'-nucleotidase activity in native thyroid plasma membranes clearly exhibited a break at 28 degrees C. Biphasic nature of Arrhenius plot showed that the enzyme activity was influenced by physical state of membrane bilayer, although phospholipids were not obligatory cofactor for this enzyme.