EXPRESSION AND INTERCONVERSION OF NEURON-SPECIFIC ENOLASE IN PATIENT SERA AND EXTRACTS FROM SMALL-CELL LUNG-CANCER CELLS

The isoforms of gamma-enolase were characterized in serum from patient s with small-cell lung cancer (SCLC) and in extracts from SCLC cell li nes and malignant melanoma tumor tissue. Large variations in the expre ssion of the 3 gamma-isoforms of enolase were observed. These forms pr obably represent the homodimeric gamma gamma-enolase, the heterodimeri c alpha gamma-enolase and the monomeric forms of gamma-enolase. Only t he dimeric forms are enzymatically active. The predominant gamma-enola se in the cell lines is the heterodimeric alpha gamma-enolase. The SCL C cell lines can be divided into two groups: one with negligible gamma gamma-enolase expression and considerable amounts of the nonneuronal alpha alpha-enolase and a second group with a large fraction of gamma gamma-enolase concomitant with a low expression of alpha-enolase. Simi lar patterns are observed in tissue extracts from malignant melanoma. When changing buffer conditions by increasing the ionic strength and d ecreasing the Mg2+ concentration, interconversions between the isozyme s occur. In contrast to the predominant alpha gamma-enolase in extract s from cell lines, the multiple forms of gamma-enolase in serum might be caused by a subunit exchange facilitated by the low Mg2+ concentrat ion in plasma. However, there seems to be a stable equilibrium between the isoforms in undiluted patient serum. The induction of subunit exc hange by perturbation in ionic strength and/or Mg2+ concentration indi cates a need for caution when choosing diluents for use in assays for neuron-specific enolase.