Rate of hydrolysis of the phosphate esters of B vitamins is reduced by zinc deficiency: In vitro and in vivo.

Extracellular hydrolysis of the phosphate esters of B vitamins (B1, B2, and B6) is crucial for their cellular uptake and metabolism. Although a few zinc-dependent enzymes have been implicated in these processes, their exact mechanisms of action remain largely unknown. This study investigated the potential involvement of phosphate group hydrolyzing enzymes in the hydrolysis of B vitamin phosphate esters. We evaluated enzyme activity in membrane lysates prepared from cells transiently transfected with these enzymes or those endogenously expressing them. Specifically, we investigated how zinc deficiency affects the rate of hydrolysis of B vitamin phosphate esters in cellular lysates. Assessment of the activities of zinc-dependent ectoenzymes in the lysates prepared from cells cultured in zinc-deficient conditions and in the serum of rats fed zinc-deficient diets revealed that zinc deficiency reduced the extracellular hydrolysis activity of B vitamin phosphate esters. Furthermore, our findings explain the similarities between several symptoms of B vitamin and zinc deficiencies. Collectively, this study provides novel insights into the diverse symptoms of zinc deficiency and could guide the development of appropriate clinical strategies.

Early secretory pathway-resident Zn transporter proteins contribute to cellular sphingolipid metabolism through activation of sphingomyelin phosphodiesterase 1.

Sphingomyelin phosphodiesterase 1 (SMPD1) converts sphingomyelin into ceramide and phosphocholine; hence, loss of SMPD1 function causes abnormal accumulation of sphingomyelin in lysosomes, which results in the lipid-storage disorder Niemann-Pick disease (types A and B). SMPD1 activity is dependent on zinc, which is coordinated at the active site of the enzyme, and although SMPD1 has been suggested to acquire zinc at the sites where the enzyme is localized, precisely how SMPD1 acquires zinc remains to be clarified. Here, we addressed this using a gene-disruption/reexpression strategy. Our results revealed that Zn transporter 5 (ZNT5)-ZNT6 heterodimers and ZNT7 homodimers, which localize in the compartments of the early secretory pathway, play essential roles in SMPD1 activation. Both ZNT complexes contribute to cellular sphingolipid metabolism by activating SMPD1 because cells lacking the functions of the two complexes exhibited a reduced ceramide to sphingomyelin content ratio in terms of their dominant molecular species and an increase in the sphingomyelin content in terms of three minor species. Moreover, mutant cells contained multilamellar body-like structures, indicative of membrane stacking and accumulation, in the cytoplasm. These findings provide novel insights into the molecular mechanism underlying the activation of SMPD1, a key enzyme in sphingolipid metabolism.

Pigmentation and TYRP1 expression are mediated by zinc through the early secretory pathway-resident ZNT proteins.

Tyrosinase (TYR) and tyrosinase-related proteins 1 and 2 (TYRP1 and TYRP2) are essential for pigmentation. They are generally classified as type-3 copper proteins, with binuclear copper active sites. Although there is experimental evidence for a copper cofactor in TYR, delivered via the copper transporter, ATP7A, the presence of copper in TYRP1 and TYRP2 has not been demonstrated. Here, we report that the expression and function of TYRP1 requires zinc, mediated by ZNT5-ZNT6 heterodimers (ZNT5-6) or ZNT7-ZNT7 homodimers (ZNT7). Loss of ZNT5-6 and ZNT7 function results in hypopigmentation in medaka fish and human melanoma cells, and is accompanied by immature melanosomes and reduced melanin content, as observed in TYRP1 dysfunction. The requirement of ZNT5-6 and ZNT7 for TYRP1 expression is conserved in human, mouse, and chicken orthologs. Our results provide novel insights into the pigmentation process and address questions regarding metalation in tyrosinase protein family.