Members of this family of 30 species of small trees in four genera are found in tropical regions of America and Africa. Some were formerly included in the Passifloraceae.
Carica papaya L. is widely grown for its edible fruit known as papaw or papaya. It should not be confused with the fruit of Asimina triloba Dunal, (fam. Annonaceae) which is also known as papaw.
The fruit of Carica papaya and other species is a source of proteolytic enzyme, which is responsible at least in part for the irritant properties of the unripe fruits. The seeds of several, perhaps all, species of Carica L. and Jarilla Rusby (which is monotypic) are also irritant; they contain a thioglucoside that releases benzyl isothiocyanate, an irritant mustard oil, when the plant material is crushed.
This genus of 21 species occurs in the warmer regions of the Americas. The occurrence of the thioglucoside glucotropaeolin in the seeds is considered to be of chemotaxonomic value since, within this family, this compound has only been found to occur in this genus and in Jarilla Rusby, but not in Jacaratia A.DC. The two tropical African species comprising the fourth genus in this family (Cylicomorpha Urb.) have not been studied. Benzyl isothiocyanate, the irritant mustard oil produced from glucotropaeolin by the action of the enzyme myrosinase, has been found to occur in the seeds of the following species (Ettlinger & Hodgkins 1956, Gmelin & Kjaer 1970, Tang et al. 1972):
This species is widely cultivated for its edible fruit. The leaves and unripe fruit contain a milky juice rich in proteolytic enzymes. The concentration of these enzymes in the unripe fruit reaches a maximum when the fruit is fully grown, and decreases as the fruit ripens, apparently disappearing as full maturity is reached (Watt & Breyer-Brandwijk 1962).
The milky sap of the plant is caustic; the juice of the ripe fruit is said not to be irritant (Arnold 1968), but Fasal (1945) states that eating the fruit can cause irritation, especially in the angles of the mouth. Chopra & Badhwar (1940) assert that the juice of the unripe fruit is acrid or even vesicant.
The milky juice of the plant can produce severe inflammation of the eye (Watt & Breyer-Brandwijk 1962), and dermatitis (Souder 1963), and has been used to remove skin blemishes and warts (Gimlette 1929, Burkill 1935). Nagata (1971) records that papaya milk has been used in Hawai‘i as an application to cuts. The fresh roots have a rubefacient action (Quisumbing 1951).
Papaya latex contains at least four proteolytically active components, including papain, chymopapains A and B, and papaya peptidase A (Glazer & Smith 1971). Papain is known to release histamine from rabbit blood cells to plasma (Rocha e Silva & Andrade 1943). It produces a burning itch when injected intradermally (Arthur & Shelley 1955). Papain has proved to be an active antigen, and sensitivity has been recorded following inhalation, ingestion, injection, and local application (Osgood 1945, Nava 1969). It has been added to some cosmetic creams to smooth the skin (Kariyone 1971).
Okaya (1938) described a case of aurantiasis cutis in a girl aged 16 who had eaten two large papaya fruits daily for about a month. Otherwise asymptomatic yellow colouration was observed mainly on the palms of the hands and the soles of the feet, the colour disappearing 2 months after consumption of papaya was stopped. The colouration was believed to have been caused by cryptoxanthin, a carotenoid pigment present in papaya.
A powder of the pungent seeds has been used in Java to produce skin irritation (Burkill 1935). A thioglucoside, glucotropaeolin, is present in the seeds. In the presence of the co-occurring enzyme myrosinase and water, benzyl isothiocyanate, an irritant mustard oil, is released (Ettlinger & Hodgkins 1956, Tang et al. 1972).
The blossoms are extremely malodourous. The Indians of Peru fear to sleep beneath this tree lest they become afflicted with gangrenous sores which are attributed to the sap (Menninger 1967).
The fruit and leaves of this plant contain a latex with high proteolytic activity, from which the protease mexicain has been isolated (Castañeda et al. 1942, Castañeda-Agulló et al. 1945). The proteolytic activity of mexicain was found to be slightly superior to that of crude papain from Carica papaya L. (Castañeda et al. 1943a). Immunochemical evidence shows that mexicain and papain are similar, but different, compounds (Estrada-Parra et al. 1969).
The ripe fruits are eaten by the native population of north-western Mexico. Tookey & Gentry (1969) isolated a papain-like proteolytic enzyme (see Carica papaya L.) from both the fruits and tubers of this species. The majority of the proteolytic activity is concentrated in the rind of the fruit. The seeds release benzyl isothiocyanate, an irritant mustard oil, when crushed (Gmelin & Kjaer 1970, Tang et al. 1972).