Copper, zinc and calcium: imaging and quantification in anterior pituitary secretory granules

Abstract

The anterior pituitary is specialized for the synthesis, storage and release of peptide hormones. The activation of inactive peptide hormone precursors requires a specific set of proteases and other post-translational processing enzymes. High levels of peptidylglycine α-amidating monooxygenase (PAM), an essential peptide processing enzyme, occur in the anterior pituitary. PAM, which converts glycine-extended peptides into amidated products, requires copper and zinc to support its two catalytic activities and calcium for structure. We used X-ray fluorescence microscopy on rat pituitary sections and inductively coupled plasma mass spectrometry on subcellular fractions prepared from rat anterior pituitary to localize and quantify copper, zinc and calcium. X-ray fluorescence microscopy indicated that the calcium concentration in pituitary tissue was about 2.5 mM, 10-times more than zinc and 50-times more than copper. Although no higher than cytosolic levels, secretory granule levels of copper exceeded PAM levels by a factor of 10. Atp7a, which transports copper into the lumen of the secretory pathway, was enriched in endosomes and Golgi, not in secretory granules. If Atp7a transfers copper directly to PAM, this pH-dependent process is likely to occur in endosomes.

The Cardiac Ryanodine Receptor Provides a Suitable Pathway for the Rapid Transport of Zinc (Zn2+)

“…This has further stimulated an intensive research effort to understand the importance of Zn 2+ in a diverse spectrum of biological functions. In mammalian cells, large amounts of Zn 2+ accumulate in various cellular compartments and organelles [ 34 , 35 , 36 ], mainly as a structural or catalytic factor directly regulating a great number of protein functions. To be candidates for Zn 2+ signaling, intracellular Zn 2+ storages must have proteins for both Zn 2+ release and reuptake.…”

“…In mammals, they are mostly localized to the plasma membrane [ 20 , 21 , 22 , 23 , 24 , 25 , 26 ] or in membranes of intracellular organelles, such as the ER and Golgi apparatus [ 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. All these intracellular compartments obviously accumulate Zn 2+ [ 34 , 35 , 36 ], but only some could be implicated in Zn 2+ signaling, exemplified by Zn 2+ transients and waves [ 35 , 37 ]. Although, there is no clear evidence, it can be proposed that the SR in cardiomyocytes acts as a dynamic storage for Zn 2+ cations, readily available for release [ 8 , 38 ].…”

Quantitative Ubiquitinomics Revealed Abnormal Ubiquitinated ATP7A Involved in Down-Regulation of ACTH in Silent Corticotroph Adenomas

“…The second step, catalyzed by the peptidylglycine amidoglycolate lyase (PAL) domain, is the zinc-dependent cleavage of the N-C-alpha bond, producing the alpha-amidated peptide and glyoxylate ( 26 ). Several metals are involved in the secretory granule formation of anterior pituitary ( 27 ). The accumulation and depot of growth hormone and prolactin in somatotrope and lactotrope secretory granules is separatively promoted by zinc ( 28 ).…”

The Long Goodbye to Glycine-Extended Peptide Hormones

“…The result as of May 2017 is that PubMed since 2013 has registered seven original articles of which four were published in 2014 [10][11][12][13], two in 2015 [14,15] and one in 2016 [16]. None of the recent publications describes or identifies specific receptors or receptor mechanisms for glycineextended peptide hormones.…”