The Related Companies, a major New York City-based developer with $12 billion in real estate, has agreed to pay $505 million to buy a chain of upscale health clubs. Equinox Holdings operates high-end fitness clubs in New York, Chicago, Los Angeles, San Francisco and South Florida.

Equinox began as a New York City-based business, but the chain has opened 21 new clubs since 2000. Private equity firms North Castle Partners (NCP) and J.W. Childs Associates (JWC) bought Equinox in 2000.

“Equinox reflects North Castle’s strategy of investing in mid-sized businesses catering to consumer demand for products and services that promote healthy living,” says NCP managing director Douglas Lehrman.