A comparative study of some physico-chemical properties of alkaline phosphatase of a human cell line, the EUE, with high level of enzyme and one of its clonal derivatives the E6D, with low activity, has been carried out.
Electrophoretic analysis reveals a multiple banding pattern within each line and qualitative differences between the two lines. The alkaline phosphatase activity of the E6D cell extracts is almost completely inhibited by 5 X 10+ M inorganic phosphate while in the EUE the enzymic activity is reduced to one third under these conditions. The enzymes of the two lines show also a different thermostability which is not referable to extrinsic factors, as demonstrated by mixing experiments. The time course of heat inactivation at 70Β°C suggests molecular heterogeneity in each line, and a prevalence of a thermostable fraction in the cells with low activity and a thermolabile one in those with high enzymic levels. A rough estimate of inactivation constants does not rule out the possibility that the molecular species in the two lines are the same but in different proportions. The cytological analysis con6rms the relationship between the number of small acrocentric chromosomes and alkaline phosphatase levels. The significance of the biochemical data in relation to the proposed model of a gene dosage effect is discussed.
A positive correlation has been found between the number of chromosomes of group 21-22 and the levels of alkaline phosphatase in clonal derivatives of the EUE strain (De Carli et al., '63, '64). The data supported the hypothesis that chromosomes of that group carry genes controlling the synthesis of alkaline phosphatase. This hypothesis was originally based on the observation of high levels of that enzyme in leukocytes of individuals carrying trisomy 21 (Alter et al., '62).
The relationship between number of chromosomes in group 21-22 and alkaline phosphatase levels has been confirmed in successive determinations made on the EUE strain and clonal sublines after two years of culture. However, we still are not able to established whether chromosome 21 or 22 or both are involved in the control of the alkaline phosphatase character. Nor can we exclude the possibility that some members of the 13-15 group may be involved in the control of the same character, since a parallel reduction of these chromosomes is frequently observed when the small acrocentric chromosomes are lost.