* To whom correspondence should be addressed.
Received May 5, 1999; Revision received November 3, 1999
The iron storage protein ferritin was purified to electrophoretic homogeneity from cow uterine myometrium. Its Mr did not exceed 440,000 and H-chains predominated in the subunit composition; the iron saturation was 43 iron ions per protein molecule. The uterine myometrial ferritin was a potent natural modulator of Ca-calmodulin-independent phosphodiesterase (Ca-CM-independent PDE, EC 126.96.36.199) isolated from the same tissue. Addition of iron-poor ferritin from uterine myometrium and iron-reach liver ferritin caused three- and two-fold inhibition of the enzyme activity, respectively. The iron transport protein transferrin in iron-saturated and iron-depleted forms can also inhibit Ca-CM-independent PDE activity by two-fold. In both cases, the degree of saturation with iron was not crucial for the inhibitory effects of these proteins on the enzyme activity. These data suggest that iron homeostasis proteins can modulate the cyclic nucleotide level in non-nervous tissue via interaction with enzymes involved in cyclic nucleotide hydrolysis.
KEY WORDS: transferrin, ferritin, cyclic nucleotide phosphodiesterase, uterine myometrium, iron