Mechanisms of caeruloplasmin biosynthesis in normal and copper-deficient rats

Abstract

To examine the mechanisms of holo-caeruloplasmin biosynthesis, we measured the serum caeruloplasmin concentration and oxidase activity, hepatic caeruloplasmin mRNA content and hepatocyte caeruloplasmin biosynthesis and secretion in normal and copper-deficient rats. Copper deficiency resulted in a near-complete loss of serum caeruloplasmin oxidase activity, yet only a 60% reduction in serum caeruloplasmin concentration and no change in the abundance of hepatic caeruloplasmin mRNA or the rate of caeruloplasmin biosynthesis. Both interleukin-1 alpha and lipopolysaccharide increased hepatic caeruloplasmin mRNA content and caeruloplasmin biosynthesis in normal and copper-deficient animals, but neither mediator increased caeruloplasmin oxidase activity in the copper-deficient group. Pulse-chase studies in primary hepatocytes from normal and copper-deficient rats revealed that the secretory rates for newly synthesized caeruloplasmin were identical, despite little or no holo-caeruloplasmin synthesis in hepatocytes of copper-deficient rats. We conclude that hepatocyte copper content has no effect on hepatic caeruloplasmin-gene expression or caeruloplasmin biosynthesis and that the incorporation of copper into newly synthesized caeruloplasmin is not a rate-limiting step in the biosynthesis or secretion of the apoprotein from rat hepatocytes.

Plasma peptidylglycine alpha-amidating monooxygenase (PAM) and ceruloplasmin are affected by age and copper status in rats and mice

“…Though Cp synthesis is not dependent on cellular copper the amount of holo-Cp secreted by liver is lower following copper deficiency (Gitlin et al, 1992). Thus measurement of the amine oxidase activity of Cp does reflect copper status.…”

“…The vast majority of plasma copper is represented by the cuproenzyme ceruloplasmin (Cp) also known as ferroxidase. Even when dietary copper is limiting, liver is fully capable of synthesizing and secreting Cp protein into the plasma after it is fully glycosylated (Holtzman and Gaumnitz, 1970;Gitlin et al, 1992). However, following dietary copper restriction plasma Cp activity is reduced due to lack of the cofactor copper.…”

Biochemical basis of regulation of human copper-transporting ATPases

“…It has been previously reported that the biosynthesis of secreted coppercontaining enzymes, such as ceruloplasmin, correlates well with the rate of copper incorporation into these proteins (44). Although the rate of apo-protein production and secretion is not affected by copper levels, the amount of holo-protein is greatly diminished when copper is limiting (45). The tight link between the copper transporter activity and the biosynthesis of copper-dependent enzymes is also emphasized by correlation in their expression levels and developmental co-regulation (for example, see (2,46)).…”

Mechanisms of Copper Incorporation into Human Ceruloplasmin

“…Ceruloplasmin is synthesized in hepatocytes and secreted into the plasma following the incorporation of six atoms of copper in the secretory pathway (8,9). Copper does not affect the rate of ceruloplasmin synthesis or secretion; however, failure to incorporate this metal during synthesis results in secretion of an unstable apoprotein that is devoid of oxidase activity and rapidly degraded in the plasma (10,11). In Wilson disease the absence or impaired function of a copper-transporting ATPase disrupts copper movement into the secretory pathway, resulting in the decreased serum ceruloplasmin observed in affected patients (1).…”