Human inosine triphosphatase: catalytic properties and population studies

Area of Science:

• Biochemistry
• Enzymology
• Cell Biology

Background:

• Inosine triphosphate pyrophosphohydrolase (ITPase) plays a role in nucleotide metabolism.
• Understanding ITPase catalytic properties and distribution is crucial for cellular function.
• Previous studies have not fully elucidated the characteristics and variability of human ITPase.

Purpose of the Study:

• To determine the catalytic properties of human erythrocyte ITPase.
• To investigate the subcellular localization and tissue distribution of ITPase activity.
• To analyze the age-dependent decay and inter-individual variability of ITPase in erythrocytes.

Main Methods:

• A reliable 2-step colorimetric assay was employed to measure ITPase activity.
• Subcellular fractionation of cultured lymphoid-line cells was performed.
• ITPase activity was measured in various human cell types, including erythrocytes, leukocytes, and fibroblasts.
• Erythrocytes were fractionated by age to determine enzyme decay kinetics.

Main Results:

• Human erythrocyte ITPase specifically hydrolyzes ITP, dITP, and XTP, with IDP as a competitive inhibitor.
• ITPase activity is primarily located in the cytosol of lymphoid cells.
• Significant variation in ITPase activity was observed across different cell types (4.9–294 U/mg protein).
• Erythrocyte ITPase activity decays exponentially with cell age, exhibiting a half-life of 35 days.
• A correlation between erythrocyte and leukocyte ITPase activity was found at lower activity ranges.
• Two individuals with near-absent erythrocyte ITPase activity were identified, suggesting it's an individual characteristic.

Conclusions:

• Human erythrocyte ITPase exhibits specific substrate hydrolysis and is predominantly cytosolic.
• ITPase activity is highly variable among individuals and cell types, and decreases with erythrocyte age.
• Low or absent ITPase activity is an intrinsic individual trait that can manifest in various cell types.