Study of pyruvate kinase activity in human astrocytomas - Alanine-inhibition test revisted

Background: Recent studies have confirmed that alterations in the isoenzyme of pyruvate kinase (PK) provide tumor cells with selective growth advantage.
Aims: Our aim was to establish the mean activity of the enzyme PK in human astrocytomas and to look for any trends in the activity with relation to histological grade.
Materials and Methods: The PK (EC 2.7.1.40) activity was measured in the tumor homogenate by spectrophotometric rate determination. ΔAbsorbance at 340 nm (A_340nm ) per minute was obtained using the maximal linear rate for both the test and the blank. Enzyme activity was estimated in the presence and absence of amino acid alanine.
Results: The mean PK level in astrocytomas was 3.5 ± 2.0 mmol/min/mg protein, which was significantly higher (24%; P < 0.001) when compared to 2.8 ± 0.3 mmol/min/mg protein in control brain. Highest PK activity was noted in grade 2 astrocytomas. In controls there was no change in PK activity in the presence of alanine. In grade 2 astrocytomas there was 7% decrease in mean PK activity in the presence of alanine, this difference in grade 3 astrocytomas was 33% and in grade 4 astrocytomas it was 61%. As the tumors were becoming malignant there was a graded increase in the levels of PK inhibition.
Conclusions: Mean PK activity was significantly higher in astrocytomas. There was a graded increase in level of PK inhibition as the tumors were becoming more malignant.

Keywords
Alanine inhibition, astrocytomas, human brain, isozymes, pyruvate kinase