Origin of the increased activity of beta-glucuronidase in the soluble fraction of rat mammary tumors during ovariectomy-induced regression.

The activity of beta-glucuronidase in methyl-N-nitrosourea-induced rat mammary tumors regressing after ovariectomy was studied. beta-Glucuronidase, acid phosphatase, and beta-hexosaminidase were similarly increased in the lysosome-rich fraction of regressing compared to growing tumors, whereas in the soluble fraction, a 5-fold increase in the activity of beta-glucuronidase midway regression was not paralleled by an increase in the specific activity of the other two enzymes. Results of sedimentability studies indicated an equal stability of the lysosomes at the two tumor conditions. The elevated beta-glucuronidase activity in the soluble fraction was completely precipitable by rabbit monospecific antisera against beta-glucuronidase from other species. The activity in the soluble and in the lysosome-rich fraction showed a similar pH dependence and electrophoretic mobility of immunoreactive subunits, and only minor differences in affinity towards concanavalin A:Sepharose. Thus, the increased "soluble" beta-glucuronidase activity is neither tumor nor cytosol specific. Cell death during tumor regression is believed to occur according to a controlled sequence of event, called apoptosis. Dying cells become apoptotic bodies which are further degraded during phagocytosis by neighboring tumor cells. We discuss that breakage of these bodies during homogenization of the tumor is the cause for the observed increase in soluble beta-glucuronidase activity, while the lysosomes of the ingesting tumor cells remain intact. Physiologically, the enhanced intratumoral availability of beta-glucuronidase and other lysosomal enzymes might facilitate the hydrolysis of conjugates of a variety of xenobiotics and, possibly, also of steroid hormones.