The smooth endoplasmic reticulum (SER) is primarily involved in the synthesis and secretion of lipids, including:

* Steroid hormones: These are produced by the adrenal glands, ovaries, and testes.

* Phospholipids: These are the primary components of cell membranes.

* Glycolipids: These are involved in cell signaling and recognition.

* Triglycerides: These are the primary form of energy storage in the body.

While the SER plays a role in lipid metabolism, it does not directly assemble and secrete proteins like the rough endoplasmic reticulum (RER) does. The RER is studded with ribosomes, which are the sites of protein synthesis.

Therefore, the macromolecule assembled and secreted by the smooth endoplasmic reticulum is primarily lipids.