Related Experiment Videos
Blood platelet heterogeneity: evidence for two classes of platelets in man and rat
Abstract:
The platelet population of man and rat can be divided into two classes of about equal size on the basis of presence/absence of an acid phosphatase which acts on para-nitrophenylphosphate (a PNPase), at pH 5. The cytochemical reaction product is in the platelet cytoplasmic matrix, without apparent association with organelles or membrane systems. We could not relate differences in staining to differences in function: all cells responded the same to activation by thrombin, ADP, or collagen, in fibrinogen binding to activated platelets, by endocytosis of fluid-phase tracers, and in internalization of latex particles. With respect to possible physiological substrates for the PNP-ase, there was no reaction product from beta-glycerophosphate, AMP, ADP, ATP, GTP, CMP, IMP, cAMP, creatine phosphate, and inositol phosphates, and the enzyme was not inhibited by 40 mM lithium. There was reaction product from tyrosine phosphate suggesting that the physiological substrate for PNP-ase is tyrosine phosphate. In rat bone marrow, megakaryocytes also were of two classes, PNPase positive and PNPase negative, suggesting that different classes of platelets arise from different classes of megakaryocytes.
Insights
Platelets can be divided into two types based on the presence or absence of para-nitrophenylphosphatase (PNPase). This enzyme, likely acting on tyrosine phosphate, does not affect platelet function or origin from distinct megakaryocytes.
Area of Science:
- Hematology
- Biochemistry
- Cell Biology
Background:
- Platelets are crucial for hemostasis and thrombosis.
- Heterogeneity within platelet populations is increasingly recognized.
- Acid phosphatases in platelets have not been well characterized.
Purpose of the Study:
- To investigate the presence and characteristics of an acid phosphatase in human and rat platelets.
- To determine the substrate specificity and localization of this platelet enzyme.
- To explore the potential relationship between enzyme activity and platelet function or origin.
Main Methods:
- Cytochemical staining for acid phosphatase activity at pH 5 using para-nitrophenylphosphate.
- Assessment of platelet responses to agonists (thrombin, ADP, collagen) including fibrinogen binding, endocytosis, and particle internalization.
- Testing of various phosphate compounds as potential enzyme substrates.
- Analysis of megakaryocytes in rat bone marrow for enzyme activity.
Main Results:
- Human and rat platelets were divided into two approximately equal populations based on para-nitrophenylphosphatase (PNPase) activity.
- The enzyme's reaction product was localized in the platelet cytoplasm, not associated with organelles.
- No functional differences were observed between PNPase-positive and negative platelets upon activation.
- Tyrosine phosphate, but not other tested phosphate compounds, served as a substrate for PNPase.
- Rat megakaryocytes also exhibited PNPase-positive and negative populations.
Conclusions:
- A distinct population of platelets possesses para-nitrophenylphosphatase (PNPase) activity, likely acting on tyrosine phosphate.
- This enzymatic difference does not correlate with functional responses or platelet activation.
- The presence of distinct megakaryocyte classes suggests a potential origin for these two platelet populations.