Cellular internalization, transport, and esterification of iodine-125-NP59 by MA-10 Leydig tumor cells. The present studies were directed toward understanding the cellular processing of the cholesterol analogue, NP59. NP59 readily entered MA-10 Leydig tumor cells. The cholesterol analogue entered the cells by binding to the plasma membrane and becoming internalized along with plasma membrane cholesterol. Internalized NP59 was readily esterified to NP59 ester. Transport of NP59 within the cell was indistinguishable from transport of cholesterol. Cholesterol and NP59 transport were under the control of cAMP, however, only cholesterol entered the mitochondria and was converted into progesterone. Thus, internalized NP59 could not be removed from the cell by conversion into steroid hormones. Esterified NP59 was metabolically inert and could not be converted back to free NP59 and free fatty acid. Since NP59 was not a substrate for the cholesteryl ester hydrolase, it became trapped in the cell as NP59 ester. 