Expression of the ENTPD5/mt-PCPH onco-protein and overexpression of the normal ENTPD5/PCPH protein contribute to the malignant transformation of diverse mammalian cell types, and PCPH is mutated and/or deregulated in various human tumor types. Expression of PCPH or mt-PCPH caused similar phenotypes, yet the effects promoted by mt-PCPH expression were consistently and substantially greater. ATP depletion and increased stress‑resistance are phenotypes commonly associated with PCPH and mt-PCPH expression. It was suggested that the intrinsic nucleoside triphosphate diphosphohydrolase (NTPDase) activity of PCPH and mt-PCPH may be responsible for these phenotypes, but direct supporting evidence remains to be established. Results from experiments designed to test such hypothesis demonstrate that, as expected, mt-PCPH expression in human colorectal carcinoma (CRC) cells decreased their ATP levels and conferred resistance to oxaliplatin, a colorectal cancer-relevant chemotherapeutic agent. Using a combination of site-directed mutagenesis, immunoprecipitation methods, in vitro enzyme activity assays and in situ enzyme activity determinations in live cells, this report also demonstrates that the mt-PCPH oncoprotein lacks detectable NTPDase activity, indicating that direct ATP cleavage by mt-PCPH did not cause the ATP depletion observed in mt-PCPH-expressing CRC cells. These results strongly suggest that the mt-PCPH oncoprotein may regulate the cellular energy levels and subsequent chemoresistance by an NTPDase-independent mechanism. Understanding possible alternative mechanisms will be essential to devise strategies for the successful treatment of predictably therapeutically resistant tumors expressing either increased PCPH levels or, particularly, the mt-PCPH oncoprotein.