You can read more about leptin on p. 745 of your textbook. The main chemical classes of hormones are as follows. Steroid hormones are enzymatically synthesized from cholesterol. Other lipids , such as prostaglandins, can also serve as hormones. Finally biogenic amines are hormones that are enzymatic modifications of amino acids.
Target cells are the cells with receptors to which a particular hormone can bind.
Cells without receptors for a particular hormone cannot respond to that hormone. Receptors are proteins. There are two major forms of hormone action on target cells. They reflect the lipid solubility of different hormones. Steroid hormones bind to nuclear receptors .