Can the Prince of Asturias act as regent?

Apparently, yes, according to Spain's Constitution:


Article 59  [Minority, Incapacity]
(1) When the King is a minor, the King's father or mother, in their absence the oldest relative closest to succession to the Crown pursuant to the order established by the Constitution, shall immediately exercise the Regency during the King's minority.
(2) If the King becomes incapable of exercising his authority and this incapacity is recognized by the Parliament, the Prince heir to the Crown shall immediately begin to exercise the Regency if he is of age.  If he is not, the procedure outlined in the previous paragraph will be adhered to until the Prince heir reaches adulthood.
(3) If there is no person who can exercise the Regency, it shall be appointed by the Parliament and shall be composed of one, three, or five persons.
(4) In order to exercise the Regency, it is necessary to be Spanish and of age.
(5) The Regency shall be exercised through constitutional mandate and always in the name of the King.