Hyaluronidases are a family of enzymes that catalyse the degradation of hyaluronic acid. Karl Meyer classified these enzymes in 1971, into three distinct groups, a scheme based on the enzyme reaction products. The three main types of hyaluronidases are two classes of eukaryotic endoglycosidase hydrolases and a prokaryotic lyase-type of glycosidase.
In humans, there are five functional hyaluronidases: HYAL1, HYAL2, HYAL3, HYAL4 and HYAL5 (also known as SPAM1 or PH-20); plus a pseudogene, HYAL6 (also known as HYALP1). The genes for HYAL1-3 are clustered in chromosome 3, while HYAL4-6 are clustered in chromosome 7.
Hyaluronidases are hyaluronoglucosidases (), i.e. they cleave the (1→4)-linkages between N-acetylglucosamine and glucuronate. The term hyaluronidase may also refer to hyaluronoglucuronidases (), which cleave (1→3)-linkages. In addition, bacterial hyaluronate lyases () may also be referred to as hyaluronidases, although this is uncommon.